Aids to Microscopic Inquiry. 45 



Without intending to supply a treatise on physics, we shall 

 commence by adverting to a few considerations, founded on 

 physical laws, which are necessary to be entertained before the 

 various phenomena exhibited under the microscope, by organic 

 bodies, can be rightly understood. 



All the creatures inhabiting our globe are, in the first place, 

 under the dominion of gravitation. Every particle of which 

 they are composed gravitates toward the centre of the globe, 

 with a force proportioned to the quantity of matter it contains. 

 The pound of feathers, the still bulkier pound of air; the 

 pound of flesh, the still denser pound of bone, etc., tend 

 downwards with precisely the same force. Each substance 

 has its own specific gravity ; that is to say, a square inch, or 

 any other given mass of it, weighs a certain and constant 

 weight. If put into water it displaces its own bulk of that 

 fluid, and the quantity of water thus displaced by a given 

 weight of any material will depend upon the density of the 

 substance employed. If, for example, we forced an ounce 

 of air into a cask of wine, and allowed an equal bulk of 

 wine to run out, the quantity of the latter would be con- 

 siderable ■ while an ounce of gold thrown into the cask 

 would only displace an insignificant drop. ee A cubic inch of 

 water weighs 814*75 times as much as a cubic inch of air; 

 both being at the temperature of 60°, and under a pressure 

 measured by a column of 30 inches of quicksilver whose tem- 

 perature is 32V* 



It is obvious that the higher the specific gravity of an 

 organism, the greater will be the exertion required to move 

 any given bulk of it. The muscular power which enables a 

 man to move his own body in walking or running would be 

 quite inadequate to his locomotion if his solids and fluids 

 weighed on the average as much as platina or gold. Thus the 

 muscular power of any creature intended to walk, to crawl, or 

 to swim, must be proportioned to the specific gravnry of its 

 components, and to the resistance afforded by the medium in 

 which it is to live. A human body will float in water so long* as 

 the chest is distended with air, and drowning usually occurs be- 

 cause the sufferers have not learned how to balance themselves 

 in the fluid so as to keep their heads above the liquid level. 

 If a man were made as big as a whale and extended over 

 seventy feet of earth, he would be perfectly helpless, because 

 his strength would be disproportioned to his bulk. His two 

 legs could make nothing of such a mass ; but give him the 

 whale's ability to live in water, and provide for respiration by 

 coming to the surface at short intervals, and his condition 



Apjohn Manual of the Metalloids, p. 192. 



