THE DEVELOPMENT OF METEOROLOGY. 
39 
tion of several different vapors, such as those that produce 
rust and those that are exhaled from the earth, the water, 
vegetables, and animals. Indeed, the springiness of the air 
excited his suspicion that there might be some vital substance 
diffused through the atmosphere. The experiments that he 
proposed to have made — that he in fact began, and that were 
carried out by his contemporary, John Mayow — bore on 
respiration, oxidation, and evaporation as the sources of new 
kinds of air. 
Boyle was the first to suggest that the atmosphere con- 
sisted of air, properly so called, and water in a state of ex- 
pansion, together with other gases that emanate from the 
earth and exert an injurious influence on the health. De 
Saussure seems to have been the first to measure the absolute 
quantity of aqueous vapor in a given volume of atmosphere. 
In 1760 Lord Cavendish showed that the vapor evaporating 
from water in a vacuum had a definite elastic pressure, which 
he measured at several different temperatures. 
The temperature of the dew-point seems to have been first 
observed by Le Roy (1750), Dalton (1800), and Daniel 
(1820). The psychrometer, or wet- and drv-bulb thermom- 
eter, is generally ascribed to August (1825), but the wet 
bulb was used long before by Beaume, and it was August 
who gave us an acceptable, rational theory of its action, while 
at the same time, and quite independently, Ivory in 1822, 
Espy in 1829, Belli in 1830, and Apjohn in 1834, intro- 
duced modifications, all of which are now combined in Fer- 
rel’s, Grassman’s, and others’ theories and tables for the 
whirled psychrometer. The relative humidity was first ob- 
served by means of a catgut hygrometer by Brander (1650), 
but the hair hygrometer of De Saussure (1780) and his per- 
sistent researches were the first steps in modern hygrometry. 
The discovery of carbonic acid gas, or fixed air, is gener- 
ally attributed to Joseph Black, of Edinburgh, who, however, 
had several predecessors less widely known. In 1752 he 
discovered that this gas is the same as choke damp, or fixed 
air. According to Ramsey, Black showed that the common 
air of the atmosphere contains a small amount of fixed air. 
