238 THE POPULAR SCIENCE MONTHLY. 



progress in the arts. They also must have means to compel Kature 

 to reveal her secrets, -^neas succeeded in his great enterprise by 

 plucking a golden branch from the tree of science. Armed with this 

 even dread Charon dared not refuse a passage across the Styx ; and 

 the gate of the Elysian fields was unbarred when he hung the branch 

 on its portal. Then new aspects of Nature were revealed : 



*' Auother sun and stars they know 

 That shine like ours, but shine below." 



It is by carrying such a golden branch from the tree of science that 

 inventors are able to advance the arts. In illustration of how slowly 

 at first and how rapidly afterward science and its applications arise, 

 I will take only two out of thousands of examj)les which lie ready to 

 my hand. One of the most familiar instances is air, for that surely 

 should have been soon understood if man's unaided senses are sufficient 

 for knowledge. Air has been under the notice of mankind ever since 

 the first man drew his first breath. It meets him at every turn ; it 

 fans him with gentle breezes, and it buffets him with storms. And 

 yet it is certain that this familiar object — air — is very imperfectly 

 understood up to the present time. We now know by recent researches 

 that air can be liquefied by pressure and cold ; but as a child still looks 

 upon air as nothing, so did man in his early state. A vessel filled 

 with air was deemed to be empty. But man, as soon as he began to 

 speculate, felt the importance of air, and deemed it to be a soul of the 

 world upon which the respiration of man and the godlike quality of 

 fire depended. Yet a really intelligent conception of these two essen- 

 tial conditions to man's existence, respiration and combustion, was not 

 formed till about a century ago (1775). No doubt long before that 

 time there had been abundant speculations regarding air. Anaximenes, 

 five hundred and forty-eight years before Christ, and Diogenes of 

 Apollonia, a century later, studied the properties of air so far as their 

 senses would allow them ; so, in fact, did Aristotle. Actual scientific 

 experiments were made on air about the year 1100 by a remarkable 

 Saracen, Alhazen, who ascertained important truths which enabled 

 Galileo, Torricelli, Otto de Guericke, and others at a later period, to 

 discover laws leading to important practical applications. Still there 

 was no intelligent conception as to the composition of air until Priest- 

 ley in 1774 repeated, with the light of science, an empirical observation 

 which Eck de Sulbach had made three hundred years before upon the 

 union of mercury with an ingredient of air, and the decomposition of 

 this compound by heat. This experiment now pi'oved that the active 

 element in air is oxygen. From that date our knowledge, derived 

 from an intelligent questioning of air by direct experiments, has gone 

 on by leaps and bounds. The air, which mainly consists of nitrogen 

 and oxygen, is now known to contain carbonic acid, ammonia, nitric 

 acid, ozone, besides hosts of living organisms which have a vast influ- 



