364 
On the Measure of Temperature, 
[Dec. 
Second Case* 
The Thermometer having its bulb covered with silver leaf. 
Excess of temp, 
of the therm. 
Total rate of 
cooling. 
Rate of cool- 
ing. in vacuo. 
Rate of cooling 
due to the air. 
200° 
6»,93 
1 (, ,50 
5°, 43 
180 
6 ,02 
1 ,23 
4 ,79 
160 
5 ,19 
1 ,00 
4 ,19 
140 
4 ,32 
0 ,80 
3 ,52 
120 
3,50 
0,62 
2 ,88 
100 
2 ,80 
0 ,48 
2 ,32 
We see in comparing the last column in each of these tables, that the corre- 
sponding members differ by very small quantities, which may be fairly attributed to 
the errors of the experiment. Air carries off then, all else being equal, the same 
quantity of heat from surfaces, whether of glass or metal. 
The two following table? contain the particulars of a similar comparison made 
with hydrogen gas, the small theremometer being substituted for the large one. 
The experiment was made at a temperature of 20°, the density being ,74 metres. 
First Case. 
The Thermometer with its natural surface. 
Excess of temp, 
of the therm. 
Total rate of 
cooling. 
Rate of cool- 
ing in vacuo. 
Rate of cooling, 
due to the con- 
tact of hydrogen. 
80° 
22°, 96 
5°, 03 
17,93 
60 
16 ,14 
3 ,54 
12,60 
40 
9 ,87 
2 ,18 
7,69 
20 
4 ,28 
0 ,95 
3,33 
Second Case. 
The Thermometer having its bulb covered with silver leaf. 
Excess of temp. 
Total rate of! 
Rate of coo\-\ 
Rate of cooling! 
of the therm. 
cooling. 1 
ing in vacuo.) 
due to hydrogen. 
80® 
19°, 59 
1°,77 
17,82 
60 
13 ,97 
1 ,29 
1 2)68 
40 
8 ,62 
0 ,87 
7.75 
20 
3 ,74 
0 ,37 
3,37 
This comparison gives for hydrogen a similar result as for air. The simi- 
larity of action being then established, in the case of surfaces which differ so much 
as glass and silver in their radiating properties, and for gases differing so much as 
air and hydrogen, it is natural we should generalise this result, and deduce from 0 
the following law : 
The loss of heat occasioned by the contact of a gas is ( all else being equal) *»' 
dependent of the state of the surface of the body which is subjected to the cooling 
process. 
This remarkable law in the communication of heat, has been already admitted 
by Mr. Leslie ; but this able philosopher has only advanced it as a conclusion 
rendered probable by the result of two indirect experiments, from which it appear 5 
that the state of the surface has only a feeble influence on the total cooling effect, 
when the circumstances are such as leave but a small share in the loss of heat at- 
tributable to radiation. As for example, when a heated body is exposed to a vio - 
lent wind, or when it is immersed in a liquid. But however ingenious such expe- 
riments may appear, they can never supply the place of the direct investigation, tor 
the case in question, where is the difficulty of supposing that air in motion ma> 
. 1,lv V sonit; property or quality which may not be predicable of still air ? Such an o >- 
jection would appear still stronger if we adinitj as Mr. Leslie supposes, that still air 
receives heat from bodies by two different means,' that is to say, by conduction as m 
