286 HISTORY OF SCIENCE. 



oil congealed. The upper fixed point was given by the temperature of 

 the human body, and the interval between was divided into 1 2 degrees, 

 and, by divisions equal to these, the scale was extended upwards. By 

 the scale of this instrument Newton found that water boiled at 34, that 

 tin melted at 7 2, etc., and, in fact, he measured a number of tempera- 

 tures. To remedy the inconvenience of the comparatively small dila- 

 tion of oil, Amontons, a French physicist, returned to the air-thermo- 

 meter, which he constructed of about 4 feet in length, but which had 

 the disadvantage of requiring its reading to be corrected by a barome- 

 trical observation. But Amontons had found that the temperature of 

 boiling water was constant, and he selected this as a fixed point on his 

 scale. But the first practically useful and accurate thermometers were 

 constructed by a scientific instrument maker of Dantzig, named 

 GABRIEL FAHRENHEIT (1686 1736). He at first used alcohol as the 

 liquid with which to fill his tubes ; vbut afterwards adopted quicksilver 

 as in almost every respect preferable". The lower fixed point on his 

 scale was the point at which the mercury stood in the tube when 

 plunged into a mixture of sal-ammoniac and snow, in what proportions 

 are not stated ; but he appears to have considered this as the lowest 

 temperature that could be obtained. The higher point was that at 

 which water boils. The interval between these he divided into 212 

 equal parts, a number which appears arbitrary, but which he was in- 

 duced to select possibly because the melting-point of ice and the 

 temperature of the human body, two other important fixed tempera- 

 tures, would fall at the exact number of degrees 32 and 96 : most of 

 the thermometers he made were not graduated beyond 96. Fahren- 

 heit's thermometer was immediately adopted in England and Germany, 

 and is still in common use in those countries. But when REAUMER 

 proposed the temperature of melting ice for the lower point in the 

 scale, and that the division of the interval of temperature up to boiling 

 water should be divided into 80 degrees, his scale was adopted by 

 France and Russia. Finally, in 1741, CELSIUS, a professor at Upsal, 

 divided the same interval into 100 equal parts, and this Centigrade 

 scale superseded Reaumer in France, and is now that preferred all 

 over the world for scientific observations. 



The world being now in possession of a scientific instrument for 

 the measures of temperatures, it was not long before it was turned to 

 account in the discovery of the true laws of heat, as a short account 

 of the experiments of Black will show. JOSEPH BLACK (1728 1799) 

 was born at Bordeaux, his father, who there engaged in commerce, 

 being a native of Belfast, but of Scottish descent. At twelve years 

 of age Black was sent to a grammar school at Belfast, and, after four 

 years spent there, to the University of Glasgow. Here he became so 

 fond of the study of chemistry that Dr. Cullen, who then taught that 

 subject, found in him an enthusiastic assistant; but Black, having 

 chosen the profession of medicine, removed to Edinburgh to finish his 



