important to add that once the higher boiling fraction was removed from the excess of sulphur trioxide, or which is the same, from the low-boiling fraction, no solid was deposited.

Most of the mixtures of pyrosulphuryl chloride and silicon oxy-chloride obtained by us crystallized at various temperatures, the highest one being -60°, the lowest -78°; some of them did not crystallize, but merely solidified to a vitreous mass, near -120°. Pyrosulphuryl chloride melts at 37°, silicon oxychloride near -40°. A mixture of equal parts of the prepared pure substances melted at -40° to -38°, but on mixing 15.6 grams of the former with 5.2 grams of the latter, and heating half of the mixture on the Bunsen flame, it was found that the heated portion took twenty times longer to crystallize than the unheated one. As stated in the introduction, the only possible explanation is that the two substances form a compound under the influence of heat. All the mixtures were obtained by means of one or more distillations, and submitted to the heat of a flame, and this influence of heat explains the phenomena of crystallization observed. In the mixtures containing approximately one molecule of each substance, the two substances are combined, and not being free, cannot crystallize when a seed of either substance is introduced; in the mixtures containing an excess of pyrosulphuryl chloride, the silicon oxychloride is all combined, but some of the pyrosulphuryl chloride remains free, hence this mixture can crystallize on a seed of pyrosulphuryl chloride, although here again, it will not crystallize on a seed of the oxychloride. The deposit formed on standing over the summer was placed in a Gooch crucible, washed twice with silicon tetrachloride, pressing down the material with a glass rod, and using a suction pump to remove all the liquid: all as rapidly as possible. The solid was then packed and sealed in tubes, in which it remained without alteration. This material appears perfectly dry; it smokes strongly in the air and attracts moisture rapidly. With water it reacts violently; a few bubbles of gas escape, and a slight yellow color (due to chlorine) develops; the white particles become transparent, but retain their original shape; no visible amount of silica separates out from the liquid. The reaction with dilute sodium hydroxide is the same, but more violent. When moistened with chloroform the solid becomes translucent and filled with bubbles. Heated over the flame it evolves white fumes, and leaves behind a white ash, which no longer reacts with water. The deposit is not homogeneous; the silicon percentage varied between 6 and 10; the portions which had been formed against the wall of the flask contained the higher percentage. One sample contained:

The study of this solid suggests that it is silica enclosing much sulphur trioxide and some silicon oxychloride and pyrosulphuryl chloride. This silica would have been formed by the equation, which has been discussed before,

Our reasons for contending that silico-phosgene is not formed have been fully stated in the introduction.

SUMMARY,

Melted sulphur trioxide and silicon tetrachloride are miscible; on standing a long time or on heating 6 to 10 hours to 50° a reaction takes place:

2SiCl4 +2SO3 = Si2OCIË + S2O5Cl2;

an excess of sulphur trioxide causes a further reaction:

The most significant result, as regards the relation of carbon and silicon, is the non-formation of silico-phosgene.

We take pleasure in acknowledging a grant from the Cyrus M. Warren Fund of Harvard University, with which the expense for the liquid air was met.

CHEMICAL LABORATORY, HARVARD UNIVERSITY.

Proceedings of the American Academy of Arts and Sciences.