Soon after the publication of Dr Black's essay, Mr Meyer, an apothecary at Osnaburgh, published a dissertation, in order to explain the differences be tween limestone and quicklime, the caustic and mild alkalies. His conclusions were quite different from those of Dr Black. According to him, limestone combines in the fire with a peculiar acid, which he called the acidum pingue, or causticuni. To this acid it- owed the peculiar acrid properties which it acquired by calcination. Alkalies had a stronger af: flinty for this acid than lime. Hence, when potash and quicklime are mixed, the lime loses its acid, and. becomes tasteless and insoluble in water ; while the alkali unites with the acid and becomes caustic.
Meyer's hypothesis being contrasted with Dr Black's theory, which soon became known in Germany, oc casioned a violent controversy, which lasted some years. Jacquin, botanical professor at Vienna, pub lished a Latin dissertation in defence of Dr Black's doctrine in 1769. This was opposed, in 1770, by Dr Crans, physician to the of Prussia, who defended the hypothesis of Meyer in a very elaborate treatise. In 1774, Lavoisicr published a treatise on the subject. He repeated the experiments of Dr Black and his disciples, and confirmed them in every particular. Since that period, Dr Black's theory of causticity, and his doctrines respecting fixed air, have been universally admitted by the chemical pub lic.
Dr Black's essay drew the attention of philoso phers to the elastic fluids, and in particular to fixed air, which he had'shewn to act so important a part in nature.. Dr Macbride of Dublin was the first who wrote on the subject. His essay appeared in 1764. He examined the fixed air evolved during fermentation, and pointed out its properties in re tarding putrefaction. But it was Mr Cavendish who first examined the properties, and pointed out the characters of fixed air. His essays on the subject were published in 1766 and 1767. Dr Priestley fol lowed soon after, and laid open the different elastic fluids in succession. Thus Dr Black's discovery is not only important in itself, but it acquires addition al value, because it led' the way to pneumatic che mistry, and was therefore the foundation of the complete revolution which the science underwent.
2. The paper on the freezing of boiled water, published in the Philosophical Transactions, is very short, and requires but little notice. He found, that water which has been newly boiled always freezes sooner than common water. The reason, he says, is, that boiled water cannot be cooled lower than 32 , without beginning to freeze ; whereas common water may be cooled several degrees lower, without losing its fluidity. This difference he ascribes to the boiling, which has deprived this water of its air. Hence, when exposed to the air, that elastic fluid begins to enter, and occasions a constant agitation, which, though imperceptible to us, is yet sufficient to prevent the water from being cooled lower than 32° without freezing. This explanation is simple and ingenious, and probably correct. Though some of the experiments of Sir Charles Blagden, on the cooling of water below the freezing point, without losing its fluidity, appear at first sight incompatible with, it.
3..The analysis of the Geyser and Reykum waters, which Dr Black published in the second volume of the Edinburgh Transactions, is one of the finest specimens of the analysis of mineral' waters which has yet appeared. Dr Black has contrived to throw an interest on his subject, of which one would hardly have conceived.that it would have admitted. This appears in a very striking point of view, when corn pared with the essays of Bergman, Klaproth, Four croy, Vaiiquelin, or any other of the consummate masters of the art of analysing mineral waters. The interest which we take in the analysis of Dr Black, is much greater than in that of any other person, The analysis is remarkably correct, and the methods fol lowedare the best that we are yet acquainted with ;
. though his directions for preparing the filtres can scarcely be followed at prescrit. His mode was te dious ; and his reliance upon the filtre much greater than it ought to have been. Chemists at present seldom trust to experiments made by weighing fil tres. They are so apt to change their weight, and, their tendency to absorb moisture is so great, that they cannot be easily weighed in a delicate balance. .A. filtre, weighing thirty grains, if dried at the fire, will absorb half a grain of moisture before you can weigh it.
4. The only part of the posthumous work, en titled Dr Black's Lectures, that requires animadver sion, is his doctrine of latent heat, contained in the first part of that book. This doctrine he brought to maturity while in Glasgow. He read an account of it to a philosophical society in that city, in the year 1762. Ever after it was taught by him with great care, and sufficiently in detail, constituting in deed one of the most important and interesting parts of his course. This theory is not only in itself highly beautiful and valuable, as it constitutes the basis of the whole doctrine of heat, one of the most instructive branches of chemical science ; but it has another claim upon our attention,—it led to Mr Watt's improvement of the steam engine, which has pro duced such mighty changes in our system of milling and in our manufactures, and which has so enor mously increased the powers and the resources of man. Dr Black's two discoveries hold a most inte resting position ; the first occasioned a complete re volution in the science of chemistry ; the second an equally complete revolution in some of our most im portant arts and manufactures. The theory of latent heat, which Dr Black deduced from an attentive ob servation of some of the most common phenomena of nature, and from some experiments of Fahrenheit, may be stated in a few words. When ice melts, it combines with a quantity of heat which enters into it, and remains in it without increasing its tempera ture. Hence it was called latent. When water freezes, it parts with the whole of this latent heat ; and it cannot freeze till it does so. Hence the slow ness of the processes of freezing and melting. Wa ter then is ice, combined with a certain quantity of heat. In like manner, when water is converted into steam, it combines with a quantity of heat ; and when steam is condensed into water, it parts with a quantity of heat. In general, when solids are con verted into liquids, they unite with a quantity of la tent heat ; and when liquids are converted into elas tic fluids, they likewise combine with latent heal. This theory was deinonstrated by the most obvious, but decisive experiments, and it was applied, with much sagacity, to explain some of the most import ant phenomena of nature. In his experimental inves tigations of the quantity of latent heat in different bodies, Dr Black was much assisted by his two ce lebrated pupils, Mr Watt and Dr Irvine. He first suggested also the curious fact, that bodies differ in their capacity for heat ; that is to say, that the same quantity of heat does not produce the same change of temperature upon different bodies. Thus it rc ' quires much more heat to raise a pound of water 100 degrees, than is necessary to produce the same change of temperature on a pound of mercury. This subject was carried much farther by Dr Irvine, who made some curious discoveries respecting it, and even.. applied it to explain Dr Black's great discovery of latent heat. His explanation was adopted by several ingenious philosophers ; and there are still some per sons who consider Dr Irvine's explanation as prefer able to Dr Black's. Dr Black himself stated some objections to Dr Irvine's explanation, with his usual modesty, which appear to be fatal to it. Dr Craw ford afterwards investigated the capacity of different bodies for heat with much industry and success, and founded on it his ingenious explanation of the source of animal heat.