1878.] 



the Influence of Light upon Protoplasm. 



211 



with the entrance tube of the pump, while the other had bored in it 

 four holes into which the ends of the experimental tubes were pushed 

 until the "shoulder" (see fig. 3) was firmly thrust against the india- 

 rubber. All junctions were luted with viscid glycerine, and it was 

 found that a good vacuum could then be produced and maintained for 

 a considerable time. 



In order conveniently to seal off the tubes they were again drawn 

 out below the shoulder, so that when complete they had the shape 

 given in the figure (fig. 3). 



When atmospheres of special composition were required the mode 

 of procedure was somewhat different ; one of the four holes in the 

 outer caoutchouc stopper was then appropriated to a gauge, formed of 

 a straight piece of tubing of sufficient length, dipping under mercury : 

 into another hole was fitted a glass tube to which was attached a piece 

 of india-rubber tubing with a clamp. The pump was then worked 

 until the gauge showed the required tension, when the gas was admitted 

 from a small gasholder by attaching the stop- cock of the gasholder to 

 the india-rubber tubing and opening the clamp. 



The nitrogen used was prepared by removing the oxygen from atmo- 

 spheric air, either by the prolonged action of alkaline solution of 

 pyrogallic acid, or, in some instances, by the combustion of phos- 

 phorus ; in the latter case the oxides of phosphorus were removed by 

 agitating with solution of caustic potash. 



Our oxygen was made by heating pure chlorate of potash alone in a 

 tube of hard glass ; lest any trace of ozone or chlorine should be 

 present the gas was slowly bubbled through solution of iodide of 

 potash ; this precaution, however, appeared to be superfluous, the 

 iodide solution remaining colourless. 



Postscript. Received October 18, 1878. 



The oxidation of hydrogen by light, demonstrated in the case of 

 oxalic acid, naturally suggests an inquiry into the deportment of 

 oxygen towards hydrogen in sunlight under other conditions. 



We have not, for the present at least, an opportunity of examining 

 this question in the detail which it demands, but we think that it may 

 be of interest to append to our paper the following brief observations. 



One of the best known facts in the chemistry of light is the combi- 

 nation effected between chlorine and hydrogen, and in their behaviour 

 towards hydrogen under the influence of light the halogens form an 

 interesting series. Thus, while chlorine and hydrogen unite explosively 

 in sunlight, bromine and hydrogen are with difficulty, if at all, induced 

 to combine, and iodine and hydrogen do not unite at all. Again, water 

 may be decomposed with the aid of sunlight both by chlorine* and by 



* Cl 2 + H 2 = 2HCl + 0. 



