- 
DA REPORT—1862. 
as the screens absorb or reflect heat: a screen of tinfoil will not allow a deposit to 
be formed ; but if the screen be of brown age there will be an abundant furthest 
deposit. So also if a bottle have attached to it disks and rings of tinfoil, paper of 
various colours, &c., no deposit will be formed in and about such disks, because 
they keep the bottle warm by preventing radiation, and even by absorbing heat. A 
disk of black paper put on a deposit already formed will clear away a much larger 
space than tinfoil will do. 
The author found that crude camphor was more sensitive in its action than 
refined; but that the experiments succeed with ordinary camphor, Borneo cam- 
phor, artificial turpentine camphor, camphoric acid, iodine, naphthaline, chloral, 
water, alcohol, ether, &c. 
Exrcrricity, Magnetism. 
On the Mechanical Power of Electro-Magnetism, with special reference to the 
Theory of Dr. Joule and Dr. Scoresby. By James Croxt, Glasgow. 
In an article by Dr. Joule and Dr. Scoresby on the mechanical power of Electro- 
magnetism*, it is stated that when the electro-magnetic engine is set in motion and 
the current in consequence reduced from a to 6, the heat manifested in the circuit 
is reduced from a? to 67, but the heat which is produced by the oxidation of the zine 
is only reduced from a to b; hence they conclude that the quantity of heat equal 
a—b produced by the zine plates, but which does not appear in the circuit, is con- 
sumed in the production of mechanical effects. That this conclusion is not satis- 
factory will appear, the author thinks, from the following considerations, viz. if we 
reduce the current from a to 6 by merely reducing the consumption of the zinc from 
a to b, the heat evolved in the circuit will in this case also be reduced from a? to 6%, 
The question now arises, what becomes of the amount of force a—6 which disap- 
pears in the circuit here also? It is not consumed in work, for no mechanical effect 
takes place. Hence, from the disappearance of heat when the electro-magnetic 
machine is set in motion, we are not warranted to conclude that it went to produce 
mechanical effects ; for it equally disappears in the other case when no meine] 
effect is produced. The true explanation of the matter, he thinks, is this : when we 
reduce the current from a to b, we reduce the heat evolved in the conducting wire 
from a? to b’, but we only reduce the heat evolved in the entire circuit from a to b; 
hence there is no disappearance of heat whatever. The simple fact is, the heat 
which is missing in the conductor will be found in the battery; however, when 
the engine is in motion there will be a deficiency in the total heat evolved equal to 
the thermal equivalent of the mechanical work performed. When the engine being 
at rest the current is equal 6, the total heat evolved is also equal 6; but when the 
current is reduced to b by the motion of the machine, the total heat evolved will 
then be equal 6—x; x being the equivalent of the mechanical work performed. 
The value of zx, therefore, is not determined by the theory of Dr. J oa and Dr. 
Scoresby. 
Let us consider the theory in relation to the origin of the mechanical work. 
When the current is equal 6, without mechanical work being performed, the heat 
evolved in the conductor is 6?; when the current is 6, and mechanical work per- 
formed, according to the theory the heat evolved in the conductor is also equal 6, 
In this case there is no reduction of heat in the conductor corresponding to the me~ 
chanical effect produced; for the heat is as great when the mechanical effect takes 
place as when it does not, being in both cases equal 67. This would lead to the 
conclusion that the mechanical effect is not derived from the current 6, for it could 
not possibly produce its full equivalent of work, in the shape of heat 6? and x 
amount of work in addition. The work xz must, therefore, according to this theory, 
be derived directly either from the chemical action in the battery or from the heat 
evolved. That it is not directly dependent upon chemical action is evident from 
* Philosophical Magazine, June 1846, 
