200 Prof. A. B. Macallum. On the Nature of the [Mar. 3, 
Amongst the more recent observers, Boveri* holds that the material in which 
the silver precipitate occurs is not a specific substance, either cement or other- 
wise, but only the contact surfaces of two neighbouring cells. He claims that 
he observed, inside of blood vessels, red blood corpuscles lying in contact with 
each other, and between the contact surfaces the silver precipitate occurred 
as between endothelial cells. What the precipitate itself is he does not say. 
Rablf regarded it as probable that the reagent combines with the albumin 
to form a silver-nitrate proteid, through union of the molecules, analogous 
to that process which operates in the precipitation of urea with mercuric salts. 
That the brown granules of the precipitate are not metallic silver is shown 
by their solubility in sodinm thiosulphate. According to Mannt the 
precipitate is probably a proteid in combination with chlorides and carbonates. 
From this review it will be seen that there is as yet much uncertainty as 
to the nature of the reaction which silver nitrate undergoes in tissues under 
the influence of light, some observers holding that the silver compound is a 
mixture of a chloride and an albuminate, both of which become coloured 
when exposed to the light, while others postulate the presence of an 
albuminate compound only. 
The cause of this confusion lies in the fact that our knowledge of the 
action of organic compounds on silver salts under the influence of light is 
very indefinite and fragmentary. It is, of course, known that when a silver 
salt is added to a solution of one of certain organic compounds, and the 
mixture exposed to hight, a more or less coloured product soon appears, the 
formation of which is supposed to be due to reduction. This is a term which 
apphed to a salt of silver, has a wide meaning, one application comprehending 
that decomposition of the salt in which metallic silver is set free, another 
involving that change of the salt in which the quantity of the element or 
substance combined with the silver is diminished. Both of these types of 
reduction are illustrated in the case of a salt of silver in association with 
organic compounds. 
The reduction to the metallic condition occurs when alkaline silver 
solutions come in contact with living protoplasm (Loew and Bokorny),§ and 
it occurs also when certain organic compounds in alkaline solution—eyg., uric 
acid, levulose, dextrose, hydroxylated benzol derivatives, hydrazine and 
aldehyde compounds—are heated with nitrate of silver. In neutral solutions 
* “ Beitrage zur Kenntniss der Nervenfasern,” ‘Abhandl. d. Kon. Bayer. Akad., Math.- 
Phys. Cl.,’ vol. 15, p. 421, 1886. 
t “Ueber geschichtete Niederschlage bei Behandlung der Gewebe mit Argentum 
nitricum,” ‘Sitzber. Wiener Akad., Math.-Phys. Cl.,’ vol. 102, Abth. 3, p. 342, 1893. 
{ ‘Physiological Histology, Methods and Theory,’ Oxford, 1902, p. 266. 
§ ‘Chemische Ursache des Lebens.,’ Miinchen, 1881. 
