CHAP. VI., 5.] 



HEAT. SIR JOHN LESLIE. 



ur 



(653.) 

 ifluence 



i radiant 

 sat. 



(654.) 



T various 

 [cum- 



inces. 



jslie's 

 ;culiar 



npropa- 

 ttion of 

 iat. 



the instrument be placed in front of an indefinite 

 plane (such as a wall) hotter than itself the rise of 

 temperature will be precisely the same, whatever be 

 the distance at which it is presented to the heated 

 surface. 



The influence of colour on the heating of bodies 

 was considered by Leslie in an original manner. It 

 was f un( l to be effectual only when the radiations are 

 luminous. A thermometer painted black or white 

 (provided the texture of the surface be the same) 

 parts with its heat, and also absorbs the heat de- 

 rived from such a source as boiling water, in an al- 

 most equal degree. The effect depends chiefly on the 

 degree of polish or condensation of the surface. But 

 with luminous sources of heat the case is widely dif- 

 ferent. This subject had been carefully considered 

 previously to the date of Leslie's work by Sir W. 

 Herschel, who had studied the absorbing power of 

 different colours on the sun's rays. Black and white 

 form the two extremes, and Leslie availed himself of 

 this principle to construct his photometer, which cer- 

 tainly (whatever may be its defects) is an elegant mo- 

 dification of the differential thermometer. It is an 

 instrument having one ball of black, the other of 

 pellucid glass, and united by a tube of the form of 

 the letter U, containing sulphuric acid tinged red as 

 an indicator. As the texture of the surfaces of both 

 balls is the same, dark heat is equally absorbed by 

 both, and the indicating liquid remains stationary. 

 But in the sun's rays, or even in common daylight, 

 the dark ball becomes most heated ; and it is not 

 unreasonable to conclude, that when the source of 

 heat remains the same, its variations of intensity are 

 correctly shown. Leslie, however, erred in consider- 

 ing that it was applicable to measuring light differing 

 in origin and quality on a comparative scale ; and 

 this error he unfortunately persevered in, after un- 

 questionable experiments had shown its fallacy. 



The Essay on Heat contains an elaborate and 

 ingenious research into the law of cooling of bodies, 

 including the effects of mass, surface, contact of air, 

 currents of air, and likewise of inclosure of the cool- 

 ing body in successive envelopes or thin cases ; and 

 the author ingeniously compared the results of actual 

 experiment with formulae based on principles more 

 or less theoretical. But a fundamental error unfor- 

 tunately runs through all this research, and shows 

 in a striking manner the fatal influence of theo- 

 retical preconception steadily maintained through a 

 course of experimental enquiry. He starts with the 

 notion, that the presence of air is essential to the 

 propagation of Heat, generally called "radiant." In 

 fact, for radiation he usually substitutes the word 

 pulsation," and ascribes the effect of surface in mo- 

 difying the cooling of bodies to its faculty of trans- 

 mitting pulsations or tremors, more or less readily, 

 to the vehicle of the air. He was indeed compelled 

 to admit that air had a double agency ; one " abduc- 

 tive," as it draws off heat by contact and by what is 



generally called " convection," that is by currents 

 which the communication of heat itself produces ; the 

 other, " pulsatory," which corresponds to what is 

 usually termed radiation, but which Leslie persisted 

 in believing to be due to tremors propagated in air, 

 after the manner of sound, and with the same velocity. 

 In the concluding chapter of the work before us he 

 considers the cooling effect of different gases, and of air 

 of different degrees of rarefaction ; and this last ex- 

 periment might, one would have thought, have satis- 

 fied him of the fallacy of his opinion ; since, taking 

 his own numbers, when air is rarefied 1024 times, the 

 " pulsatory energy" is only diminished one-third part. 

 In fact, it appears as if his work broke off abruptly, 

 when the course of observation became irreconcilable 

 with the opinions advanced in the early part of it. 



After this analysis of Leslie's greatest contribution (655.) 

 to science, I cannot afford space to dwell upon his Hii j minor 

 minor inventions. I pass over them, however, with ^"e^in 8 "" 

 the less regret because they have been fully dwelt e xperi g 

 upon in his "Dissertation," of which the present is ament. 

 continuation, and in his articles on Cold and Meteo- 

 rology in the Encyclopedia. The most original and 

 important of these was his very beautiful process of 

 producing ice in quantity by the cold of evaporation, 

 in the receiver of an air-pump ; rendered effectual by 

 his ingenious use of absorbent surfaces for with- 

 drawing the vapour. This experiment was completed 

 in 1811, and attracted much attention. It was con- 

 nected with his researches on hygrometry, to which 

 he also adapted his differential thermometer. But 

 in the development of this difficult theory, he was 

 less successful, nor indeed could he well be so, whilst 

 he adhered to the old opinions respecting the affinity 

 of air for moisture. 



Having filled the Mathematical Chair from 1 805 to (656.) 

 1819, Leslie was in the latter year translated to that Close of his 

 of Natural Philosophy, vacant by the death of Play- career- 

 fair. He had a good collection of apparatus, and de- 

 vised many ingenious experiments. In 1820, he was 

 elected corresponding member of the Institute of 

 France, and died on the 3d November 1832, at the 

 age of 66, having received the honour of knighthood, 

 on the recommendation of Lord Brougham, but a few 

 months before. 



In closing this brief sketch of Sir John Leslie's (657.) 

 career, we cannot fail to observe the combination of 

 unusual powers with unusual drawbacks to their 

 complete and vigorous exertion. Whilst he had the i n 80me 

 chief merits, he had also the most serious defects, of points de- 

 the self-formed student. He was ardent and ambi- fective 

 tious in the pursuit of knowledge. He must have 

 been for many years a hard, if not a methodical stu- 

 dent ; he united good mathematical knowledge with a 

 real love of experiment ; he was gifted with a strong 

 memory, and confident in the exercise of all his 

 powers. Why, with so many advantages he did not 

 achieve more, nor put forth even what he did to 



