considerable number of facts have been described, showing that pressure will more or less influence such chemical actions as are accompanied by an evolution of gas, so that it may cause a compound to be permanent, which otherwise would be decomposed.” If increase of pressure retards, a diminution of pressure will facilitate such chemical changes, so that one can only explain the results which I have obtained, on the ground that many boiled fluids, which will not undergo change when protected from the influence of atmospheric particles (living or not living) at the same time that they are subjected to ordinary or increased pressure, will, on the contrary, pass through such changes when pressure is removed, and the fluids are preserved _in vacuo_. It is not pretended that this is a rule applicable to _all_ organic fluids--far from it. Diminution of pressure does seem, however, to be a very potential cause of change in some fluids. The extent to which changes of a fermentative character can progress in the absence of atmospheric oxygen, is also evidently subject to much variation, in accordance with the nature of the dissolved fermentable substances. [84] ‘On the Direct Correlation of Mechanical and Chemical Forces.’--Proceedings of the Royal Society, vol. xii. pp. 539 and 546. These facts are not so new and exceptional, however, as they may at first sight appear. It has been long known that a boiled fluid extremely prone to change, will not yield infusoria if the vessel in which it is contained is filled with the fluid. Burdach says[85]:--“Gruithuisen a reconnu que des infusions, même très fécondes d’ailleurs, celles du foin par exemple, ne donnaient point d’infusoires dans des flacons de verre dont le bouchon était assez enfoncé pour toucher à la surface de l’eau.” On the other hand, no experiments with which I am acquainted, in which heated fluids and calcined air have been shut up in closed flasks, have yielded so many positive results as those of Professor Wyman of Cambridge, U.S.,--and his were performed under precisely the reverse conditions. Large flasks were used, and only 1/15–1/20 of their bulk was filled by the experimental fluids.[86] Gruithuisen’s results were explained by Burdach on the ground that a certain amount of air was necessary, and it was also with the view of subjecting his fluids to as large an amount of air (calcined) as possible, that Professor Wyman employed small quantities of fluids in large flasks. These views were dictated by the chemical doctrines of Gay-Lussac and others, to the effect that the oxygen of the air is the initiator or _primum movens_ of fermentative changes.[87] [85] ‘Traité de Physiologie.’ Translated by Jourdan. 1837. t. i., p. 18. [86] ‘American Journal of Science,’ July, 1862. [87] See Gerhardt’s, ‘Chimie Organique,’ t. iv. pp. 540 and 547.