(653.) 
Influence 
of colour 
on radiant 
heat. 
(654.) 
The laws of ; 
cooling un- 
Crap. VI., § 5.] 
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 found to be effectual only when the radiations are 
luminous. A thermometer painted black or white 
(provided the teatwre 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, 
der various including the effects of mass, surface, contact of air, 
cireum- 
stances. 
eurrents 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 formule 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 
VOL. I. 
HEAT.—SIR JOHN LESLIE. 
945 
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 His minor 
minor inventions. I pass over them, however, with 
writings— 
Freezing 
the less regret because they have been fully dwelt experi- 
upon in his “ Dissertation,’’ of which the present is a ment. 
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 1805 to 
(656.) 
1819, Leslie was in the latter year translated to that Close of his 
of Natural Philosophy, vacant by the death of Play- 
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 
career. 
(657.) 
career, we cannot fail to observe the combination of S 
unusual powers with unusual drawbacks to their °°P™°? 
complete and vigorous exertion. 
character 
Whilst he had the jy some 
chief merits, he had also the most serious defects, of points de- 
the self-formed student. 
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 
6D 
He was ardent and ambi- fect 
ve 
