438 Dr.J.^.Draiper on the Distribution of 



Admitting the correctness of the theorem that the molecules 

 of most elementary gases consist of two atoms, he conceives that 

 the effect of light on chlorine is to tend to divide or actually 

 to divide its molecules into isolated atoms. These atoms, if the 

 gas be kept in the dark, may reunite into molecules. 



The chlorine molecule cannot unite with hydrogen; the chlorine 

 atom can ; heuce insolation brings on combination. 



But if the cbloi'ine be unmixed, there will, as a consequence of 

 insolation, be a certain proportion of uncombined atoms; and 

 from this, together with Avogadro^s theorem, is drawn the con- 

 clusion that this gas, through insolation, increases in specific 

 volume. 



Moreover, as the reunion of the chlorine atoms probably pro- 

 duces heat, rays of high refrangibility will cause chlorine to ex- 

 pand ; but it will contract to its original volume when no longer 

 under the influence of light. 



In corroboration of this conclusion, Budde found that a diffe- 

 rential thermometerfilled with chlorine showed a certain expansion 

 when placed in the red or yellow rays ; but it gave an expansion 

 six or seven times as great when in the violet rays. With carbonic 

 acid and ether no such effect took place. 



It should not be forgotten, however, in considering the bearing 

 of these experiments, that chlorine, merely because it is yellowish 

 green, will absorb rays of a complementary (that is, of an indigo 

 and violet) colour, and become heated thereby. 



It has next to be determined whether the points of maximum 

 action (that is, the points of maximum absorption) correspond to 

 the rays of emission of either or both these gases, as they appa- 



rently ought to do under Angstrom^s law : '^A gas, when lumi- 

 nous, remits rays of light of the same refrangibility as those 

 which it has the power to absorb.'^ 



Of the four rays characteristic of hydrogen, there is one the 

 wave-length of which is 43^0. It is in the indigo space. 



Pliicker gives for chlorine a ray nearly answering to this. Its 

 wave-length is 4338 ; and also another, 4346, the latter being 

 one of the best-marked of the chlorine lines. 



There are therefore rays in the indigo which are absorbed both 

 by hydrogen and by chlorine. The place of these rays in the 

 spectrum corresponds to that in which the gases unite, the 

 place of maximum action for their mixture. 



But the absorptive action of chlorine is not limited to a few 

 isolated lines. That gas removes a very large portion of the 

 spectrum. Subsequent experiments must determine whether 

 each of these lines of absorption is also a line of maximum che- 

 inical action. 



The chlorhydrogen actino meter, referred to in previous para- 



