96 M. C. Lea—Sensitiveness of Silver Haloids. 
of prime importance for these purposes. A difference of one 
per cent in the speed of the engine sometimes cannot be toler- 
ated, and yet at another time one must have the power of 
increasing and diminishing through wide limits. The only 
motor, among many I have examined and tried, that is per- 
fectly satisfactory, is Brayton’s Petroleum Ready Motor. This 
remarkable and admirable engine acts like an instrument of 
night, supplying water and air to the aquaria in the Centennial 
Exhibition at Philadelphia. At any time on going into the 
laboratory it can be started in a few seconds, even though 1t 
as not been running for days. 
Henry Draper’s Observatory, Hastings-on-Hudson, New York. 
Art. XV.—On the Theory of the Action of certain Organic Sub- 
waging tnereasing the Sensitiveness of Silver Haloids ; by 
» G. dma. 
In the early days of photo-chemistry, it was observed that 
all the silver haloids gained in sensitiveness by being in contact 
with a soluble silver salt; that when the soluble silver salt was 
removed by careful washing, a considerable diminution in sen- 
