64 REPORT — 1856. 



A series of experiments conducted with lamp-light for the purpose of 

 testing the accuracy of the instrument, gave the following results: — 



As soon as the atmospheric air has been completely expelled from the ap- 

 paratus by the electrolytic gas, and the equilibrium between the amounts of 

 gas absorbed by the water, and the fiee gas established, an action is observed 

 on exposing the gas to the light. This action, however, does not commence 

 immediately on exposure to the light ; a short time elapses before the absorp- 

 tion of the water in the tube (d) begins, but this soon takes place, showing 

 that the combination eiFected by the light in the vessel (c) has commenced. 

 This absorption becomes gradually quicker until a certain rapidity is at- 

 tained, after which the action continues regular as long as the source of 

 light remains constant. This peculiar phsenomenon, to which we have given 

 the name of Photochemical Liduction, is one of great interest and import- 

 ance, and as the study of this branch of the subject has occupied our pai'- 

 ticular attention, the results obtained will be subsequently detailed. 



On passing more gas through the apparatus and again isolating the 

 mixture, the same phaenomenon is observed, with the difference, that the 

 constant action is larger than in the former case, that is, the gaseous mixture 

 has become more sensitive. In this way, by continuing to lead the gas 

 evolved from successive portions of hydrochloric acid through the apparatus, 

 the action brought about by a gas flame of the same dimensions increases 

 regularl)', until, after having continued the operation for several (from 12 to 

 18) hours, the amount of action effected by the flame remains constant. 

 The apparatus has then attained its maximum degree of sensibility, and, as 

 we shall show, always gives comparable results. Before this maximum 

 action is attained, upwards of 5000 cub. cent, of gas must be passed through 

 the apparatus, which contains only about 2 cub. cent, of water requiring 

 saturation. Observations made with the apparatus thus prepared, showed 

 that the light from a gas lamp concentrated by a lens produced always 

 exactly the same amount of action on various days and with fresh gaseous 

 mixtures evolved from different portions of acid. These experiments sufficed 

 to show that our apparatus was capable of producing reliable and accurate 

 results. We next determined the limits of concentration between which the 

 hydrochloric acid can be used, and experiment showed that the amount of an- 

 hydrous acid contained in solution must not diminish to 20 per cent., as the gas 

 evolved from an acid of that concentration nolonger gives the maximum action. 

 Having assured ourselves that the apparatus gave, under these circum- 

 stances, comparable results, it became necessary to examine whether the 

 heat evolved from the combination of the gases, and more especially the 

 heat radiated from the source of light, had any appreciable efl^ect upon the 

 indications. By comparing the relative volumes of the vessel, in which the 

 insolation takes place, and the absorption tube, it was found that a rise of 

 less than 0°"04; Cent, in the mass of the gas would cause an expansion of 

 1 millimetre on the absorption tube. Hence the apparatus is not only a 

 photometer, but also a very delicate air-thermometer. In order to prevent 

 any of the rays of radiant heat from expanding the gas, the insolation-vessel 

 was placed behind a double metallic screen furnished with a metallic cap 

 fitting over the vessel. The rays of light fell on the gas through an opening 

 in the screen filled by a layer of water contained between two plate-glass 

 surfaces. By filling the apparatus with atmospheric air, it was proved that 

 with this arrangement the source of light may be placed within a few inches 

 of the gaseous mixture, without the radiant heat interfering in the least with 

 the indications. The sources of exterior error arising from radiant heat 

 having been thus removed, it only remained to determine whether the heat 



