230 
Proceedings of the Royal Society of Edinburgh. [Sess. 
methanes — methyl iodide and iodoform — do not seem to form such combina- 
tions. When methyl iodide or iodoform, in bulk or in solution, is added to 
an alcoholic solution of silver nitrate, a precipitate forms as in the case of 
methylene iodide, but it is amorphous and consists of silver iodide. With 
methyl iodide Fanto {Monatsli., xxiv, 477, 1903), using special pre- 
cautions, appears to have obtained a crystalline compound (AgN 03 ) 2 . Agl. 
Donnan and his collaborators, like ourselves, only obtained silver iodide 
from the action of methyl iodide or iodoform on alcoholic silver nitrate 
solutions. 
Burke and Donnan {Journ. Chem. Soc., Ixxxv, p. 569, 1904), and 
Donnan and Potts (loc. cit), determined the velocity coefficients for methyl 
iodide and silver nitrate, and iodoform and silver nitrate in alcoholic 
solution. The last-named observers also made a few experiments with 
methylene iodide and silver nitrate under similar conditions. They found 
that the velocity coefficient is about one hundred times smaller than the 
corresponding coefficient for iodoform ; and since the velocity coefficient for 
iodoform is only about eight times smaller than the velocity coefficient for 
methyl iodide, they conclude that “ in point of reactivity with silver 
nitrate, methylene iodide does not therefore occupy a position intermediate 
between methyl iodide and iodoform, but is characterised by a very great 
sluggishness.” The experiments were made at 25° C., and the reaction 
velocities were determined by estimating the amount of silver salt in 
solution by the thiocyanate method. We have repeated some of Donnan 
and Potts’ experiments and can corroborate their facts, but in view of the 
formation of a double compound of silver nitrate and methylene iodide, 
which is moderately soluble in alcohol and only gradually decomposes in 
it, and the silver of which can be estimated by the thiocyanate method, it 
is questionable if the results obtained are strictly comparable with those 
got from methyl iodide and iodoform. The dilutions employed by them 
are not given, but were probably not greater than N/25, giving N/50 when 
the two solutions were mixed. The solubility of the double compound at 
25° C. is N/25 5 or 2*2 grns. in 100 gms. of ethyl alcohol. It is, however, 
difficult to say whether this compound exists as such in solution. Its 
silver can be precipitated by thiocyanate, which shows that some of the silver 
exists in solution as free silver ions. Its electrical conductivity in absolute 
alcohol does not differ materially from that of a corresponding solution 
of silver nitrate — the molecular conductivity of a N/25’5 solution of the 
double compound in absolute ethyl alcohol being 1'68 x 10“^, and of a 
N/25'6 solution of silver nitrate in absolute ethyl alcohol 1'6I x 10“^. 
When equal volumes of N/20 silver nitrate and N/20 methylene iodide 
