_Ether vapour_ is heavy--two and a half times heavier than air. It therefore tends in a room to flow towards the floor, and to remain for some time unmixed with the general atmosphere. Since it is highly explosive, this constitutes a definite danger if any naked light or open fire is present. Everyone who handles ether should bear in mind these physical peculiarities of its vapour. Ether is affected by prolonged exposure to bright sunlight, and also by prolonged bubbling through it of air, nitrous oxide, or oxygen. Ether which has been subjected to any of these measures should be inspected before being put back into reserve, and if any brown discoloration is noticed, it should either be sent to the hospital laboratory for redistillation, or presented to the theatre sister for use as a cleaning agent. Such contributions are always gratefully received. Most of the ordinary impurities of ether are acid in reaction, while ether itself is absolutely neutral. Any specimen which turns litmus paper red should be sent to the laboratory for examination. Sources of Supply. Ethylic alcohol may be prepared either from ethyl alcohol or from methylated spirits. In the former case it carries, however, the cost of the duty imposed upon potable spirit, and since perfectly good anæsthetic ether can be prepared from the latter source, it is waste of money to use the more expensive article. Physiology. Ether acts upon the nervous system like other anæsthetics: as compared with chloroform, however, the stage of excitation of each centre before its paralysis is apt to be marked. There is, therefore, in some subjects, a greater tendency to struggle; healthy subjects properly handled do not show much evidence of irritation of the cerebrum. All, however, show some evidence of stimulation of the respiratory centre, which is not prolonged. _Prolonged deep and rapid respiration under ether is due to other causes than the action of the drug itself._ It is of course seen in “closed ether,” but the active agent is excess of CO_{2}, not ether. The working margin of safety in ether, _i.e._ the stage between loss of spinal reflexes and the poisoning of respiratory centre is much wider than in chloroform. Some experiments of Waller made many years ago showed that upon nerve tissue, ether acts much less powerfully than chloroform: in the proportion, he found, of one to seven or eight. These laboratory results have received entire confirmation by later workers who have estimated the actual vapour strength required of either drug to produce or maintain anæsthesia. Roughly, to induce anæsthesia, we require 2 per cent. chloroform, or 16 to 18 per cent. of ether (_see_ Appendix II.). The Circulation.