3. Dr. Priestley, at a pretty early period of his chemical career, had discovered that when nitrous gas is mixed with common air over water, a diminution of bulk takes place; that there is a still greater diminution of bulk when oxygen gas is employed instead of common air; and that the diminution is always proportional to the quantity of oxygen gas present in the gas mixed with the nitrous gas. This discovery induced him to employ nitrous gas as a test of the quantity of oxygen present in common air; and various instruments were contrived to facilitate the mixture of the gases, and the measurement of the diminution of volume which took place. As the goodness of air, or its fitness to support combustion, and maintain animal life, was conceived to depend upon the proportion of oxygen gas which it contained, these instruments were distinguished by the name of _eudiometers_; the simplest of them was contrived by Fontana, and is usually distinguished by the name of the _eudiometer of Fontana_. Philosophers, in examining air by means of this instrument, at various seasons, and in various places, had found considerable differences in the diminution of bulk: hence they inferred that the proportion of oxygen varies in different places; and to this variation they ascribed the healthiness or noxiousness of particular situations. For example, Dr. Ingenhousz had found a greater proportion of oxygen in the air above the sea, and on the sea-coast; and to this he ascribed the healthiness of maritime situations. Mr. Cavendish examined this important point with his usual patient industry and acute discernment, and published the result in the Philosophical Transactions for 1783. He ascertained that the apparent variations were owing to inaccuracies in making the experiment; and that when the requisite precautions are taken, the proportion of oxygen in air is found constant in all places, and at all seasons. This conclusion has since been confirmed by numerous observations in every part of the globe. Mr. Cavendish also analyzed common air, and found it to consist of 79·16 volumes azotic gas, 20·84 volumes oxygen gas. ------ 100·00