Polytechnic Association. 873 



sions. This diffusion of gases may, therefore, be defined as the uni- 

 form intermingling of dissimilar molecules. 



If the molecules el and ed thus commingled while in the dark, be 

 exposed to direct sunlight, an instantaneous and complete chemical 

 combination occurs with explosive violence, but without condensation ; 

 or if exposed to diffused daylight, the union of elements will be 

 gradual and without explosion ; the resulting compound in each case 

 being hydrochloric acid gas. 



The affinity or force of chemism is generated by the action of light 

 on the colored gas chlorine, which, by absorbing all the rays and 

 transmitting only the yellowish green, acquires a power which seems 

 to be expended by the union of that element with hydrogen. Early 

 in the present century, M. Benard announced that the new properties 

 acquired by chlorine on exposure to light were derived from the violet 

 ray. In 1843 Draper proved by experiment the relative power of 

 each ray in producing this change, the actinic rays being altogether 

 the most effective.* Mr. E. Budde has recently described a remark- 

 able experiment in this direction. He found that a differential ther- 

 mometer filled with chlorine expanded about seven times more in the 

 violet than in the red ray of the solar spectrum ; when the same ther- 

 mometer was filled with C0 2 no action was noticed.f 



As the combination of hydrogen and chlorine is effected without 

 change of volume, it is obvious that the molecule el does not unite 

 with the molecule ed, forming a compound molecule el-ed. The con- 

 clusion is, therefore, unavoidable, that each molecule has been divided 

 into two equal parts, and that by affinity, like parts have been sepa- 

 rated, and unlike parts have been united. These parts are the smallest 

 quantities that can be isolated, and are, in fact, the atoms recognized, 

 by the chemist. If this smallest combining proportion or atom be 

 designated by a, the actual composition of the hydrogen molecule el, 

 weighing 2, may be clearly represented by al-al (weight 1 + 1), and 

 the chlorine molecule ed weighing 72, by ad-ad (weight 35*5 + 35-5). 

 As the attraction of al to al, and of ad to ad is, after exposure to 

 light, less than of al to ad, there is an instantaneous chemical change 

 by which one molecule of hydrogen and one molecule of chlorine are 

 transformed into two molecules of hydrochloric acid gas. This reac- 

 tion is clearly indicated by the following equation : 

 al-al + ad-ad = alad + alad= 2 alad. 



* A Treatise on the Forces which produce the Organization of Plants. By Prof. 

 John "William Draper. New York : Harper and Brothers. 1843. 

 fPogg. Annalen, for 1871, No. 10. 



