174 Prof. A. B. Macallum and Miss M. L. Menten. [July 24, 



the chlorine liberated by the reducing action of the sunlight from the 

 already formed chloride of silver, and this addition to the precipitate, under 

 the influence of the light, itself is reduced, thus rendering more pronounced 

 the original reaction. This serves to demonstrate the presence of infinitesimal 

 traces of chlorides, and, consequently, when we find, after the fullest pre- 

 cautions have been taken, that a tissue element or a cellular constituent 

 gives no silver reaction, we can be certain that haloids are not present, in 

 other words, such structures are absolutely free from them. 



It may be objected that, at most, only 8 per cent, of the chloride is 

 converted into the subchloride, and, therefore, if the quantity of the chloride 

 formed should be very minute, that of the subchloride resulting being not 

 more than one-twelfth of the other, might escape observation, or be invisible, 

 while the structures under consideration could not be considered to be free 

 from chlorides. This may be met hy pointing out that, if we add nitrate of 

 silver to a solution of sodium chloride corresponding to one part of chlorine 

 in 1,000,000, the opalescence resulting is distinct, and, after the action of 

 sunlight, the reduction is observable. Here one part of chlorine in 12,000,000 

 at least of water serves to demonstrate not only its own presence but also 

 that of the other 11 parts. 



On this account the reaction is the most sensitive, the most delicate one 

 that is to be found in micro-chemistry. When it is not obtained in a tissue 

 or cellular element, one may safely conclude that chlorides are wholly absent 

 from it. 



There is another reagent of which we made some use, and which we found 

 serviceable. This was a solution of mercurous nitrate. "When a quantity of 

 this is added to a solution of a chloride, there results, as is well known, a 

 white precipitate of mercurous chloride, whose solubility is 3"1 parts in 

 1,000,000 of water at 18° C, while the chloride of silver is soluble to the 

 extent of 1\7 parts in the same quantity of water at the same temperature.* 

 The mercurous chloride is, therefore, slightly more soluble than the silver 

 chloride, and the mercurous reagent is consequently less sensitive than the 

 other as a precipitant for chlorides, but against this may be placed the fact that 

 it is possible in tissues to reveal the presence of all the mercurous chloride 

 formed, while the presence of the whole of the chloride of silver pre- 



* These results were obtained by calculation from the electrical conductivity of 

 AgCl + Aq and Hg s CI 2 +Aq, by Kohlrausch and Eose (' Zeit. fur Physik. Chem.,' vol. 12, 

 p. 241). Halleman (ibid., p. 132) found 1 part of AgCl soluble in 715,800 parts of water 

 at 13 0- 8 C, and in 384,100 at 26° - 5 C. Stas, on the other hand, states (' Untersuchungen 

 iiber Chem. Proportionen,' Transl. by Aronstein, pp, 46 and 47) that 1/20, and even 

 1/100 milligramme of silver can be demonstrated in 1 litre of water by the addition of 

 sodium chloride. 



