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ftill more hidden cause which renders one fubftance better than another for confining heat. I have fhown by a direct and unexceptionable experiment, that heat can pass through the Torricellian vacuum*, though with rather more difficulty than in air (the conducting power of air being to that of a Torricellian vacuum as 1000 to 604, or as 6 to 10, very nearly); but if heat can pafs where there is no air, it muft in that cafe pafs by a medium more fubtil than air: -a medium which most probably pervades all folid bodies with the greatest facility, and which muft certainly pervade either the glass or the mercury employed in making a Torricellian

vacuum.

Now, if there exifts a medium more fubtile than air, by which heat may be conducted, is it not poffible that there may exist a certain affinity between that medium and fenfible bodies? a certain attraction or cohesion by means of which bodies in general, or some kinds of bodies in particular, may, fome how or other, impede this medium in its operations in conducting or tranfporting heat from one place to another? It appeared from the refult of feveral of my experiments, of which I have given an account in detail in my paper before mentioned, published in the year 1786, in the 76th volume of the Philofophical Transactions, that the conducting power of a Torricellian vacuum is to that of air as 604 to 1000:- but I found by a fubfequent experiment (fee my fecond paper on heat, publifhed in the Philofophical Tranfactions for the year

1792) that fifty-five parts in bulk of air, with one part of fine raw filk, formed a covering for confining heat, the conducting power of which was to that of air as 576 to 1284; or as 448 to 1000. Now, from the refult of this laft-mentioned experiment, it should seem that the introduction into the space through which the heat paffed, of so small a quantity of raw filk as one-fiftyfixth part of the volume, or capacity of that space (which now contained fifty-five parts of air and one part of filk) more impervious to heat than even a Torricellian` vacuum. The filk must therefore not only have completely deftroyed the conducting power of the air, but muft alfo at the fame time have very fenfibly impaired that of the etherial fluid which probably occu pies the interstices of air, and which ferves to conduct heat through a Torricellian vacuum: for a Torricellian vacuum was a better conductor of heat than this medium, in the proportion of 604 to 448. But I forbear to enlarge upon this fubject, being fenfible of the danger of reafoning upon the properties of a fluid whofe existence is even doubt. ful; and feeling that our knowledge of the nature of heat, and of the manner in which it is communicated from one body to another, is much too imperfect and obfcure to enable us to pursue thefe fpeculations with any profpect of fuccess or advantage.

Whatever may be the manner in which heat is communicated from one body to another, I think it has been fufficiently proved that it paffes with great difficulty through

*See my Experiments on Heat, published in the Philofophical Tranfactions, vol. 76.

confined

confined air; and the knowledge of this fact is very important, as it enables us to take our measures with certainty and with facility for confining heat, and directing its operations to useful purposes.

But atmospheric air is not the only non-conductor of heat. All kinds of air, artificial as well as natural, and in general all elaftic fluids, fteam not excepted, feem to poffefs this property in as high a degree of perfection as atmospheric

air.

That fteam is not a conductor of heat, I proved by the following experiment: A large globular bottle being provided, of very thin and very tranfparent glafs, with a narrow neck, and its bottom drawn in ward fo as to form a hollow hemifphere about fix inches in diameter; this bottle, which was about eight inches in diameter externally, being filled with cold water, was placéd in a fhallow dish, or rather plate, about ten inches in diameter, with a flat bottom, formed of very thin feet brafs, and raifed upon a tripod, and which contained a small quantity (about two-tenths of an inch in depth) of water; a fpiritlamp being then placed under the middle of this plate, in a very few minutes the water in the plate began to boil, and the hollow formed by the bottom of the bottle was filled with clouds of fteam, which, after circulating in it with furprifing rapidity four or five minutes, and after forcing out a good deal of air from under the bottle, began gradually to clear up. At the end of dight or ten minutes (when, as I fuppofed, the air remaining with the fteam in the hollow cavity formed by the bottom of the bottle, had acquired nearly the fame tempera

ture as that of the fteam) theft clouds totally difappeared; and, though the water continued to boil with the utmoft violence, the contents of this hollow cavity became fo perfectly invifible, and fo little appearance was there of fteam, that, had it not been for the ftreams of water which were continually run. ning down its fides, I should almost have been tempted to doubt whether any fteam was actually generated.

Upon lifting up for an inftant one fide of the bottle, and letting in a fmaller quantity of cold air, the clouds inftantly returned, and continued circulating feveral minutes with great rapidity, and then gradually difappeared as before. This experiment was repeated feveral times, and always with the fame refult; the fteam always becoming vifible when cold air was mixed with it, and afterwards recovering its tranfparency when, part of this air being expelled, that which remained had acquired the temperature of the fteam.

Finding that cold air introduced under the bottle caused the steam to be partially condensed, and clouds to be formed, I was defirous of feeing what visible effects would be produced by introducing a cold folid body under the bottle. I imagined that if fteam was a condutor of heat, fome part of the heat in the fteam paffing out of it into the cold body, clouds would of course be formed; but I thought if fteam was a non-conductor of heat; that is to fay, if one particle of fteam could not communicate any part of its heat to its neighbouring particles; in that cafe, as the cold body could only affect the particles of fteam actually in contact with it, no cloud would

appear

appear; and the refult of the experiment fhowed that team is in fact a non-conductor of heat; for, notwithstanding the cold body ufed in this experiment was very large and very cold, being a folid lump of ice nearly as large as an hen's egg, placed in the middle of the hollow cavity under the bottle, upon a fmall tripod or ftand made of iron-wire; yet as foon as the clouds which we formed in confequence of the unavoidable introduction of cold air in lifting up the bottle to introduce the ice, were diffipated (which foon happened) the fteam became fo perfectly tranfparent and invifible, that not the fmalleft appearance of cloudinefs was to be feen anywhere, not even about the ice, which, as it went on to melt, appeared as clear and transparent as a piece of the finest rock-cryftal.

The refults of thefe experiments compared with thofe formerly al luded to, in which I had endeavoured to afcertain the mott advantageous forms for boilers, opened to me an entirely new field for fpeculation and improvement in the management of fire. They thowed my that not only cold air, but also hot air and hot steam, and hot mixtures of air and steam, are non-conductors of heat; coniequently that the hota. pour which rifes from burning fuel, and even the flame itself, is a nonconductor of heat.

On the Danger of using Veffels of Lead,
Copper, or Brafs, in Dairies. By Mr.
Thomas Hayes, Surgeon, of Hamficad.
From the Letters and Papers of the Bath
and Weft of England Society for the
Encouragement of Agriculture, &c.

This may be thought a bold affertion; but a little calm reflection, and a careful examination of the phenomena which attend the combuftion of fuel, and the communication of heat by flame, will thow it to be wellfounded; and the advantages which may be derived from the knowledge This experiment, which I firft of this fact, are of very great importmade at Florence, in the month of ance indeed. November, 1793, was repeated feveral times in the prefence of Lord Palmerston, who was then at Florence, and Monfieur de Fontana. In thefe experiments the air was not entirely expelled from under the bottle; on the contrary, a confiderable quantity of it remained mixed with the fteam even after the clouds had totally difappeared, as I found by a particular experiment, made with a view to afcertain that fact; but that circumftance does not render the refult of this experiment lefs curious: on the contrary, I think it tends to make it more furprifing. It should seem that neither the mafs of steam, nor that of air, were at all cooled by the body of ice which they furrounded; for if the air had been cooled (in mafs) it feems highly probable that the clouds would have returned.

MA

ANY eminent physicians have afferted, that butter is very unwholefome; while others, equally eminent, have confidered it as not only innocent, but as a good afliftant to digeftion; and each have been faid to ground their opi-` nions upon experience. Perhaps both may be right; and butter may be innocent or mifchievous according as it contains many or few adventitious materials, collected from vellels, &c. ufed in the process of making it.

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I am led to thefe conjectures by obferving, that in almost all the great dairies, the milk is fuffered to ftand in lead, brass, or copper veffels, to throw up the cream. The clofeness of the texture of these metals, and their coldnefs and folidity, contribute to feparate a greater quantity of cream from the milk than would be done by wooden trundles, or earthen pans; both of which are alfo fometimes made use of.

As I wish to establish the poffibility of the fact, that milk may corrode or diffolve particles of the veffels above mentioned, and therefore be liable to communicate pernicious qualities to the butter, I beg leave to fubmit the reasons from which I draw this conclufion.

Whoever has been much in great dairies, muft have obferved a peculiarly four frowfy fmell in them, although they be ever fo well attended to in refpect of cleanlinefs, &c. In fome, where the managers are not very cleanly, this fmell is extremely difagreeable, owing moftly to the corrupted milk. In fome it arifes from the utenfils being fcalded in the dairy; and in others, from a bad conftruction of the building itself, the want of a fufficient circulation of air, water, &c. but, in all, a great deal of the lighter or more volatile parts of the milk fly off from the furface of the pans, and furnish a great quantity of acid effluvia to the furrounding air and ceiling; which is again depofited on every thing beneath it, and, of courfe, often on the veffels, after they have been put by clean, at the times of their being out of ufe. This may be obferved to give a dull fort of appearance to brafs and copper, as if you had breathed

on them; for if you rub your finger lightly over the veffels, you will have both the taste and smell of the metal.

It also happens fometimes, that after the vessels are washed, they are not carefully rinced, nor perfectly dried by the fire; fo that fome of the milk, &c. is left on the furface of them; which diffolves the metals, either by its animal, oily, or acefcent qualities.

This is not the only way, nor the worst, by which the butter may be come impregnated with mischief. The greater the quantity of cream thrown up from the milk, the larger profits accrue to the dairy-man; therefore he keeps it in the vetiels as long as he can, and it is frequently kept till it is very four, and capable of acting upon them: if they are of lead, a calx of fugar of lead is produced; if of brafs or copper, verdigris.

It is true, the quantity cannot be very great: this however will depend upon the degree of fournefs and length of time which the milk ftands; but, independent of the acid, the animal oil in the cream will diffolve brafs and copper.

That an acid floats in the atmo

fphere of a dairy, may be proved by placing therein a bafon of fyrup of violets for a little time, which will be found to turn red.

If then I am right in my conjectures, as I think I am, from the innumerable experiments and obfervations which I have made to fatisfy myself of the fact, and which it would be trifling to relate here, may not the reputation of the wholesomeness or unwholesomeness of butter depend upon, or be owing to, fome of the above causes? And may not many a casual, nay obsti

nate

L

te complaint, which phyficians ve in vain laboured to account for, ve originated from this fource? utter is found very frequently to ccafion much diforder to very eakly, delicate, and irritable ftoachs; yet these stomachs will bear live-oil. This cannot easily be acounted for, but from metallic imregnation.

I will not contend that all the ill ffects attributed to butter are caufed by the mineral particles which it gains by the means before stated: I only infift that it is poffible, and indeed very probable; and that, when butter is free from thefe particles, it is not fo unwholesome as fome have afferted; though, when it does contain them, it is found to diforder very tender perfons.

To enlarge upon the fubject, or attempt to explain the many ways by which a very fmall quantity of the above metals may prove injurious to the human frame, in fome particular constitutions, would be only to repeat what has already been faid by abler writers*. Some will perhaps fay, that my ideas are very far-fetched; and others, that my opinions are ill-founded; but, I truft, whoever has read the induftrious Researches of Sir George Baker on the Effects of Lead, and the melancholy cafe of a young lady who died in confequence of eating pickled famphire very flightly impregnated with copper, and which others ate without being difeafed, as related by Dr. Percival, will receive my opinions with lefs objection. If I have erred, I have done it in honourable company.

I fhall be very glad if the foregoing obfervations have fufficient influence on the dairy-men to induce them to change their utenfils. Very commodious veffels may be made of caft-iron, equally well fitted for the purposes of the dairy, which will not be expenfive, and will be more innocent and cleanly.

On the Ancient and Modern Navigation of India. From Dr. Vincent's Voyage of Nearchus, 4to.

I

CANNOT take a final leave of this coaft without obferving, that the whole distance from the Indus to Cape Jafk comes out as near as may be, fix hundred and twenty-five miles, equal to the estimation of Arrian on the coast of the Icthyophagi alone; and this number of miles Nearchus was from feventy to feventy-five days in paffing. If, however, with due allowance made for fetting out against the monsoon, and twenty-four days loft at Cape Monze, we reduce the whole to forty days, we may form a comparative view between ancient and mo dern navigation; for it appears from the journal of the Houghton Eaft Indiaman, that the made the fame run in thirteen days; and upon her return was only five days from Gomeroon to Scindy Bar. But fo far is this from diminishing the credit of the firft navigators, that it is enhanced by every difficulty they had to furmount;-weak veffels with inexperienced mariners; no provifions but fuch as an unknown coast might furnish; no convenience for fleep

* See Sir George Baker's papers on the effects of lead, in the Medical Tranfactions; Dr. Percival's paper in the fame; and Dr. Falconer on copper veffels.

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