644 THE POPULAR SCIENCE MONTHLY. 



independent concepts. But if cowardice and slavishness gave the 

 true and only explanation, still more pointed would be the lesson 

 taught by the modern general exchange of the same courteous 

 action between strong and weak, rich and poor. 



The history of salutations does not directly show the contest 

 of good and evil or of any principles, but it illustrates the tran- 

 sition from egoism to altruism. Whatever was a custom, men 

 considered to be right, while it lasted. Men have not at any time 

 chosen between industrialism and militarism, but an evolution 

 has proceeded in industrialism and militarism themselves as also 

 in peoples, who have advanced, though slowly and with stumbles, 

 from lower to higher planes of culture. Differing environments 

 affected their earliest conceptions and practices, and expedited or 

 delayed their march. Those peoples who have reached civiliza- 

 tion and enlightenment can still find the representatives of their 

 early greetings among remote savages, and perhaps trace some of 

 the salutations above mentioned to subhuman ancestors. Ages 

 before the great poet wrote, the human race obeyed the precept, to 



"Move upward, working out the beast, 

 And let the ape and tiger die." 



Note. A similar study of verbal salutations, inculcating the same lessons as the pres- 

 ent article on gestural greetings, has been published by the same author in the American 

 Anthropologist for July, 1890, under the title of Customs of Courtesy. 



[Concluded.] 



-*- 



NON-CONDUCTORS OF HEAT. 



By JOHN M. OEDWAY, 



PEOFESSOR OF APPLIED CHEMISTRY IN TULANE UNIVERSITY OF LOUISIANA. 



IT is a matter of common observation that a hot body continu- 

 ally gives off its heat to things around it, until at length the 

 giver and the receivers all come to a common temperature. This 

 gradual equalization may be brought about in three different 

 ways : In the first place, heat is thrown off in every possible 

 direction from every point of a heated body by what we call 

 radiation. Secondly, when air, water, or any other fluid is in 

 contact with a hot surface that is not directly over it, the touch- 

 ing particles become warm and light, and move away to give 

 place to others. This carrying away heat by the successive 

 particles of a fluid is called convection. In the third place, when 

 a solid substance is placed against anything of a higher tempera- 

 ture, its nearest parts are warmed and give up a portion of the 

 heat received to those parts lying next to them; and these, 

 again, share their gain with those next in order ; and so on, till 



