TEANSACTIONS OF SECTION B. 633 



the expansion up to that point determined by collecting and measuring the air 

 expelled on heating. From this volume the temperature is again calculated. Com- 

 munication with the gauge is then made and the apparatus quickly exhausted as 

 far as may be thought requisite. The remainder of the operation is the same as in 

 the old method already referred to, the density being found from the formula 



d = 



0-00008y58f (C-OPs ( P X ~J 



■' /b0(l + 0'00367««)'^ J 



Pi ' 7bO(l + 0'00367««; 



(s = substance ; C = capacity of apparatus ; P = initial pressure ; Pj = final pressure ; 

 i" = temperature during experiment.) 



The main objection to the above method might lie in the fact that its accuracy 

 decreases with the pressure employed (see original paper, ' Ber.' xx. 916). Experi- 

 ments having been made to find a more direct method, the following arrangement 

 was found to answer all requirements. A large Victor Meyer's apparatus, with 

 a bulb 30 ctm. long by 4 5 ctm. diameter, is used, fitted with a detachablj& 

 top-piece, communicating with a barometer tube and a large measuring tube about 

 40 ctm. long by 3 ctm. diameter, and connected with a mercury reservoir. The 

 apparatus is heated till constant, exhausted as far as may be rec^uisite, aud the 

 reservoir so adjusted that the measuring tube is completely filled with mercury. 

 The pressure, as shown by the barometer tube, is then noted, substance introduced, 

 and the surplus pressure generated is removed by lowering the reservoir till the 

 gauge again registers the initial pressure. The volume of gas in the measuring tube 

 is then determined, and from it the density calculated as in Victor Meyer's method. 

 Neither the temperature of the bath nor the exact capacity of the apparatus need 

 be known. The residts are accurate. 



An apparatus like the above, only made of suitable material, and slightly 

 adapted to suit special requirements, might be used for the study of dissociation at 

 very high temperatures and varying pressures. 



6. On Phoiographincf Hydrogen and Chlorine Bulbs hij aid of the Flash of 

 Light which caused their Explosion. By Professor P. Phillips Bedson, D.Sc. 



The notices which have appeared in ' Nature ' (vol. xxxvii. p. 576 ; vol. xxxviii. 

 p. 16) of C. du Bois Raymonde's successful attempt to photograph the expanded 

 pupil of the eye by a flash of magnesium suggested to my mind the experiment of 

 attempting to photograph a glass bulb containing a mixture of equal volumes of 

 hjdrogen and chlorine, which, as is well known, combines with explosive violence 

 and consequent shattering of the bulb when exposed to the flash of magnesium 

 light. 



At my suggestion, Mr. Saville Shaw, demonstrator in chemistry at the 

 Durham College of Science, Newcastle-upon-Tyne, undertook the experiment, with 

 the result (as shown in the accompanying photograph) that a clearly defined image 

 of the bulb can be obtained, although the bulb itself is shattered by the explosion. 



The method of procedure was as follows. An empty bulb similar to the one to 

 be exploded was clamped in a stand and accurately focussed in gaslight. The room 

 was then completely darkened, the empty bulb replaced by one containing the 

 mixed gases, and after uncovering the lens combination was induced by the igni- 

 tion of a mixture of magnesium dust and potassium chlorate placed close to the 

 bulb. In this way two bulbs have been photographed, one of which is exhibited. 

 It will be seen on close examination that, whilst the outline of the bulb itself is 

 perfectly clear, it is fainter than the jagged stem which was left unbroken, and 

 consequently exposed for a much longer time than the bulb. A small mirror was 

 attached to the bulb with the intention of reflecting the light into the camera. 

 The result, however, shows that this was quite unnecessary, and the only efiect of 

 this arrangement has been its reproduction in the photograph. 



It is proposed to make further experiments in this direction, which may perhaps 

 throw some light on the question of photo-chemical induction. 



