DISCOVERY BY MEANS OF NEW OBSERVATIONS. 571 



It was by observing the action of substances upon 

 each other, with and without the application of heat, that 

 many of the ordinary tests in chemical analysis were dis- 

 covered. Many of the discoveries also in geology, mineral- 

 ogy, anatomy, the action of medicines, &c., have resulted 

 from the ordinary use of the observing faculty. Graspard 

 Asellius, Professor of Anatomy at Pavia, discovered, by 

 observation, in 1622, the lacteal vessels, which convey food 

 to the blood ; and Jean Pacquet, in 1647, by further 

 observation discovered that those vessels pour the nutri- 

 ment into the thoracic duct. In the early part of the seven- 

 teenth century also, and whilst the celebrated anatomist 

 Harvey was his pupil, Professor Fabricius Aquapendente, of 

 Padua, discovered by observation, that many of our veins 

 contain valves, which lie open as long as the blood is 

 flowing towards the heart ; and it was a knowledge of this 

 fact which led Harvey towards his great discovery of the 

 circulation of the blood. It was largely by means of 

 observations of different kinds taken during his extensive 

 travels, that Humboldt made his numerous discoveries in 

 terrestrial physics and natural history, the tracing of 

 isothermal lines (i.e. those of equal temperature) over this 

 globe; the existence of green plants living in perfect 

 darkness in the mines at Freyberg, &c. &c. William 

 Scoresby, about the year 1816, discovered by sailing to the 

 80th parallel of latitude, that the water was free from 

 ice over a space of 18,000 square miles. Agassiz also, by 

 means largely of observation, became a great discoverer of 

 glacier phenomena, and found what were the chief signs, 

 such as erratic blocks of stone, glacial drifts, scratchings, 

 and moraines, by means of which the previous existence of 

 ancient glaciers might be inferred ; and that the northern 

 parts of Europe and America were at one time covered with 

 fields of ice. It was by observation that Brewster discovered 



