The oxygen income and carbonic acid output are the essential changes brought about by respiration, therefore the presence of oxygen in a certain proportion is absolutely necessary for life. The 21 per cent, of O of the atmosphere suffices to saturate the haemoglobin of the blood, and 14 per cent, of O has been found to be capable of sustaining life without producing any marked change in respiration.

Dyspnoea is produced by an atmosphere containing only 7.5 per cent, of O. This dyspnoea rapidly increases as the percentage of O is further decreased, and when it gets as low as 3 per cent, suffocation speedily ensues.

The output of C02 can be accomplished if the lungs be ventilated by any harmless or indifferent gas, and since the manufacture of the C02 does not take place in the lungs, its elimination can go on independently of the quantity of O in them. The 79 per cent, of N contained in the atmosphere has a passive duty to perform in diluting the O and facilitating the escape of the C02 from the lungs.

Indifferent gases are those which produce no unpleasant effect of themselves, but which, in the absence of O, are incapable of sustaining life, such as nitrogen, hydrogen, and CH1.

Irrespirable gases are such as, owing to the irritating effect on the air passages, cannot be respired in quantity, as they cause instant closure of the glottis. In small quantities they irritate and produce cough, and if persisted in, inflammation of the air passages; among these are chlorine, ammonia, ozone, nitrous, sulphurous, hydrochloric, and hydrofluoric acids.

Poisonous gases are those which can be breathed without much inconvenience, but when brought into union with the blood cause death. Of these there are many varieties. (1) Those which permanently usurp the place of oxygen with the haemoglobin, viz.: carbon monoxide (CO), hydrocyanic acid (HCN). (2) Narcotic: (a) Carbonic dioxide (C02), of which 10 per cent, is rapidly fatal, 1.0 per cent, is poisonous, and over 0.1 per cent, injurious, (B) Nitrogen monoxide (N20). Both of these gases lead to a peculiar asphyxia without convulsions, (r) Chloroform, ether, etc. (3) Sulphuretted hydrogen (H2S), which reduces the oxyhaemoglobin and produces sulphur and water. (4) Phos-phuretted hydrogen (PH2), arseniuretted hydrogen (AsH2), and cyanogen gas (C2N2) also have specially poisonous effects.