552 



B L A C K. 



BUtk. analysis is remarkably correct, and the methods fol- 

 "~""V~ ' lowed are the best thai we are yet acquainted with ; 

 though his directions for preparing the filtres can 

 scarcely be followed at present. 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 

 Ihcy 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 



only part of the posthumous work, en- 



can weigh it. 



only 

 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 6team engine, which has pro- 

 duced such mighty changes in our system of mining 

 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- 



Eortant arts and manufactures. The theory of latent 

 eat, 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 heat. 

 This theory was demonstrated 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 docs not produce the same change 

 of temperature upon different bodies. Thas it re- 



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 sever d 

 ingenious philosophers ; and there arc 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. 



Though Dr Black never himself published any 

 account of his peculiar opinions respecting latent 

 heat, they soon came to be generally known all over 

 Europe, in consequence of the great concourse of 

 students from all quarters that attended the chemical 

 class at Edinburgh. Various dissertations both respect- 

 ing latent heat, and the capacity of bodies for heat, 

 were published at different times, and in different 

 places. The authors seldom or never refer to Dr 

 Black ; but tacitly assume to themselves the merit of 

 originality. And, in consequence of Dr Black's in- 

 dolence and carelessness, it is now almost impossible 

 to determine how far these claims were well or ill 

 founded. That Dr Black was the original disco- 

 verer, and that he had taught the doctrine publicly, 

 at least ten years before any competitor appeared, is 

 perfectly established. But whether the subsequent 

 authors derived their information from students who 

 had attended Dr Black's class, or had stumbled 

 themselves upon the theory which they published, it 

 is now very difficult to say. The first publisher on 

 the subject was Mr Wilcke of Sweden, whose dis- 

 sertation on the capacity of different bodies for heat, 

 appeared in 1773. Lavoisier and Laplace published 

 their experiments on the heat necessary to melt va- 

 rious bodies, and upon the heat evolved during com- 

 bustion about the year 1777. Mr Cavendish, in a 

 dissertation published about the year 1785, claims 

 as his own the discovery of latent heat ; but says in 

 a note, that he understood that Dr Black teaches a 

 similar doctrine. As to the attempt of De Luc, in 

 his Idies Stir la Meteorologie, to claim the original 

 discovery of latent heat, it does not seem entitled to 

 any attention whatever ; and has been sufficiently 

 exposed by Mr Watt and Professor Robison. Mi 

 Cavendish, from the character of the man, and the 

 great sagacity which he possessed, would be the most 

 likely to have hit upon the theory. But it became 

 so generally known, and so much had been written 

 upon it before his claim appeared, that it is impos- 

 sible to doubt that he had imbibed his opinions re- 

 specting heat from these publications ; and certain 

 opinions respecting heat being adopted, the doctrine 

 of latent heat followed as an easy consequence. Up- 

 on the whole, Dr Black's originality admits of no 

 doubt ; and had he published an essay on the sub- 

 ject, as he did with regard to fixed air, no competi- 

 tor whatever would have appeared ; and his rank as 

 a discoverer, at least among the chemists of the con- 



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