74 Scientific Intelligence. 
experience, he prints in pclae type, on the lower half, quota- 
tions bearing directly on the subject in hand, and taken from the 
est authorities known. In this way the Soo eg of over two 
hundred authors have been secured to the reader. The science 
and the art of photography is given in twenty-seven lessons, eac 
treating of one branch. The first of these on the treatment of 
technique of the wet plate process in all its parts. med dry plate 
process follows this, and then some of the more recent photo- 
type sine: and the book - age some asctak practical 
suggestions. The work appears to be a great success in its man- 
ner as well as its matter. It ai cer sainly become the standard 
book on photography in this country. G. F. B. 
6. Conservation of Electricity.—In a memoir by M. G. Lirr- 
MANN, presented to the French Academy by M. Jamin, the author 
maintains that the quantity of matter and the quantity of energy 
ingnttadee in aatore Which remain le, The dist the 
ae nmantitg of alenteasisy wis ha a. body receives ane ids; x can 
e, for example, the potential which the body acquires, ¥ its 
capacity, or a quantity proportional to the capacity. et dm be 
the quantity of electricity received by a body when = is sacstaaed 
by ca and y by dy; one can then i dm =Pdx+Qdy, in which 
P and Q are two functions of # an y. The principle of the 
dongerentses of electricity is expr i by the condition that dm 
shall be an exact differential. Divide, for instance, any system 
in which an electrical phenomenon is produced, into two portions, | 
Se nd Let a and 6 be the simultaneous variations of these 
wo portions. In virtue of this principle of the Conservation 
of Electricity, we must have a+b=0, When A passes over 2 
closed cycle, that is to say, when its final state corresponds to its 
initial one, a=0 and ’=0. We can then write /dm=0. In 
order that /dm may be zero for every closed cycle, it is neces 
sary that dm shall be an exact differental, a ae In this 
Y de 
rwe can write the analytical expressions for the general 
ip of Ae ee ac of Electricity.— Comptes ro 
0. 18 0 
lent to that of ten to fifteen Bone elements. In obtaining, 
therefore, a light from a battery of thirty to forty Bunsen cells, 
