Though so early occupied with electricity, Volta did not confine himself to this subject, nor even to the wider field of physics, and that he did not hesitate, in his scientific inquisitiveness, to follow clues even in chemistry, is well illustrated by his first step in the investigation of gases. His attention being called to bubbles breaking on the surface of Lake Maggiore while on a fishing excursion, he set about finding their source, and noted that whenever the bottom of the lake near the shore was stirred somewhat a number of bubbles arose, and that the gas thus set free was inflammable. He constructed an electrical pistol in which gases thus set free were exploded by a spark from the electrophorus. About the same time, on the principle of the electrical pistol, he invented the eudiometer, an apparatus by means of which the oxygen content of air could be determined. With regard to these inventions, Arago calls attention to a special quality that is peculiar to all of Volta's work. "There is not a single one of the discoveries of Professor Volta," says the distinguished French scientist, "which can be said to be the result of chance. Every instrument with which he has enriched science existed in principle in his imagination before an artisan began to put it into a material shape." After these inventions and his previous work, it is not surprising that in 1774 Volta was offered the professorship of experimental physics in the College of Como. Here he labored for five years, until he received a call, in 1779, to the professorship of physics at the University of Pavia, where he was destined to remain in an active teaching capacity for a period of forty years. Volta began his life-work as professor of physics at Pavia by extending his observations on gases. He was the first to demonstrate the expansion of gases under heat, especially as regards their increased expansibility at higher temperatures. Many observers had been at work on this problem before his time, but there were serious discrepancies in the results reported. Volta was the first to point out the reasons for the apparent inconsistencies of previous investigators' findings; and from his observations alone some valuable data might have been obtained for the establishment of what has since become known as the "law of Charles." At this time, his knowledge of English enabled him to follow English discoveries closely, and he seems to have paid particular attention to the work of Cavendish and Priestley. Not long after Cavendish's description of the method of obtaining pure hydrogen, Volta made a series of observations on the relations of spongy platinum to this gas, and pointed out the spontaneous ignition that takes place when the two substances are brought together. This experiment is the basis of what has since been known as the hydrogen lamp, called, from the German observer who first made it a practical instrument, Dobereiner's lamp.