8 Thomson, Detectioji of Arse?i2C in Beer. 



exposure to the light for six weeks ; a second photograph 

 of the same, taken after exposure, is shown in Fig. 6. 



It is difficult to imagine why these arsenic mirrors 

 and deposits should have wholly or partially disappeared 

 after being sealed up in an atmosphere of hydrogen. 

 It is curious, however, that, whilst the mirror from the 

 middle tube has entirely disappeared, those in the first 

 and third tubes have only partially faded. 



Other instances of fading are shown in the photo- 

 graphs Figs. 7 and 8, all the deposits being obtained by 

 the old process, in which the tubes were not cooled 

 with water. The tubes Nos. i and 2 were unsealed, 

 3 and 4 were sealed in air, 5 and 6 in hydrogen, 7 and 

 8 in nitrogen, and 9 and 10 in carbon dioxide, the 

 mirrors being from the same amount of arsenic solution. 

 The tubes 2, 4, 6, 8, and 10 were detached from the card 

 and exposed to the light for one month, the others being 

 kept in the dark ; they were then remounted and again 

 photographed. It will be .seen in Fig. 8 that the mirrors 

 in tubes 2 and 4 (exposed to the air) and in 8 (exposed 

 to pure nitrogen prepared by heating ammonium nitrite, 

 and passing the same through a tube containing copper 

 gauze heated to redness) have faded considerably, while 

 in tubes 6 and 10 (in hydrogen and in carbon dioxide) 

 the black or second portion of the deposit only has 

 faded. The mirrors formed by the cooling process 

 appear to have suffered no change in hydrogen. 



My modified and new process affords a much more 

 accurate method of approximately estimating minute 

 quantities of arsenic, and it is much more delicate than 

 the process previously employed ; it is, in fact, .so delicate 

 that I have now failed to get any zinc which is absolutely 

 free from any trace of arsenic. A very distinct mirror is 

 formed with the ^oVuth of a grain of arsenic trioxide per 



