636 
Proceedings of the Royal Irish Academy. 
These equations enable us to eliminate the troublesome coefficients 
of attraction, and to replace them by the elegant and easily con- 
ceived geometrical ideas of angular velocity or periodic time. 
By making y8 equal to the atomic weight of fluorine, chlorine, 
bromine, and iodine, we obtain the conditions belonging to hydrofluoric, 
hydrochloric, hydrobromic, and hydriodic acids respectively. 
4. Formation of Sydrochloric Acid from Sydrogen and Chlorine. — 
Hydrogen and chlorine, when mixed together in the proportion 
necessary to form hydrochloric acid, do not combine in the dark, but 
do so explosively when exposed to sunlight, especially to the violet 
rays, or when exposed to the electric or magnesium light. They also 
combine when heated to a temperature of 150°, and the hydrochloric 
acid decomposes again into hydrogen and chlorine when heated to a 
temperature of 1500°. 
"When two molecules of chlorine and hydrogen combine they form 
two molecules of hydrochloric acid, and are subject during the trans- 
formation to two dynamical laws. 
(«) The conservation of areas. 
(5) The Law of Energy or vis viva. ^N'o matter what happens during 
the collision, the sum of the areas {moment of momenta) remains con- 
stant, and the total energy of the system before and after the shock 
differs by a quantity that can be measured by experiment. 
The conservation of areas gives the following : — Before the shock 
Hydrogen molecule = ^aPm. 
Chlorine molecule = ^^Pta^. 
After the shock — 
2 Hydrochloric molecules = 2 (a^^ + ^w^) m'. 
Equating these, we find 
(a(o + )8o)i) P = {aP + o)'; 
or, making a, oo, equal unity, and ^ = ^ we obtain finally from the 
conservation of areas 
(^+l)o,'==/3(l+M)- (4) 
From the Law of Energy we find before collision-— 
Energy of position = 2^ + /^i ^ 5 
Energy of motion = - (aw- + Paii) P-, 
