I860.] on the Chemical Action of the Solar Rays. 211 



The speaker remarked, that his object was to bring before his 

 audience the principles and mode of action of a method employed for 

 the measurement of the chemical action of light.* 



As an illustration of the chemical action of light, attention was 

 dirfected to the fact, that when a perfectly pure mixture of exactly equal 

 volumes of chlorine and hydrogen gases is exposed to light, the gases 

 combine, producing an equal volume of hydrochloric acid gas, whilst 

 no such combination occurs in the dark. This combination may occur 

 gradually, or with great rapidity. If the chemical activity of the light 

 be great, the union takes place quickly, great heat is evolved, a sudden 

 expansion takes place, and the vessel containing the mixture of chlorine 

 and hydrogen is shattered by the explosion. The gradual or slow 

 combination may be rendered evident by allowing the hydrochloric acid 

 thus formed to be absorbed by water ; the consequent diminution of 

 bulk of the gas accurately representing the chemical action effected. 



This mixture of equal volumes of chlorine and hydrogen is used as 

 the sensitive substance for measuring the chemical action of light. It 

 is evolved in the perfectly pure state by the electrolytic decomposition 

 of strong aqueous hydrochloric acid ; and it is by this method only, 

 that it can be prepared. The gases thus evolved are in the exact pro- 

 portion in which they exist in hydrochloric acid ; so that, if by any 

 means, we re-combine these gases, no trace of either substance will 

 remain behind, the whole uniting to form hydrochloric acid. 



For the purpose of measuring this chemical action, effected, not 

 only by solar light, but also by light from many artificial sources, 

 we require some instrument, which is to the chemical action of 

 light what the thermometer is to the heat actions ; an instrument 

 which will show objectively the amount of chemically active light. 

 We must be sure, in the first place, that our mode of measurement is a 

 reliable one. That, as in the case of the thermometer, equal incre- 

 ments of volume, correspond to equal increments of heat, so, in the 

 new instrument, the indications, however obtained, shall be propor- 

 tional to, and represent the amount of chemical rays emanating from, 

 any source. 



This has been accomplished in the chemical photometer ; by the 

 help of which an accurate measurement of the chemical action of light 

 is effected. 



The facts upon which this mode of measurement is based, may be 

 summed up as follows : 



1. Exactly equal volumes of chlorine and hydrogen gases, when 



mixed, combine together on exposure to light, forming 

 hydrochloric acid gas. 



2. This combination does not occur in the dark. 



♦ For a detailed description of apparatus, &c. see " Photochemical Researches," 

 Part 1. " Measurement of the Chemical Action of Light," by R. Bunsen and H. E. 

 Roscoe.— PAt7. Trans. 1857, p. 355. 



