428 Aids to Microscopic Inquiry. 



AIDS TO MICROSCOPIC INQUIRY. 



IV. — Notes on Organic Chemistry. 



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Living organisms are composed either entirely of soft sub- 

 stances, or of soft substances strengthened and. supported by- 

 harder materials. A growing tree exhibits fluids which are 

 concerned in supplying new materials, soft tissues in which 

 growth is going on, and hard tissues in which growth has been 

 completed. A living animal may be entirely soft, like one of 

 the Gregarina, spoken of in the last number, in the article on 

 Zoological Classification ; or it may contain hard parts, giving 

 support to the soft, as in the sponges, whose spicula are either 

 calcareous or siliceous ; or, as in a man, whose firm bones owe 

 their strength and density to phosphate of lime, interpene- 

 trating an organic material. The soft parts may, in the aggre- 

 gate, be described as the substantive portions of the creature, 

 while the hard parts perform the function of adjectives, qualifying 

 - — by supporting and strengthening — the soft parts in par- 

 ticular directions, or as a whole. The soft parts of organisms 

 are mainly composed of four substances, oxygen, .hydrogen, 

 nitrogen, and carbon. Of these, the first three are gases ; that 

 is to say, when separated from their compounds they exist in 

 the aeriform state under any known conditions of temperature, 

 pressure, etc. The fourth is a solid, which, in its pure state, 

 has neither been melted nor volatilized. 



Most of our readers will be acquainted with the leading- 

 properties of these four substances ; but as we intend this series 

 of papers to help beginners, as well as to suggest some useful 

 reflections to those who are more advanced, we shall say a few 

 words about each one, by way of introduction. Oxygen is a 

 widely present substance, and is essential to life, as we know it, 

 on our globe. It constitutes eight-ninths by weight of pure 

 water, which is a definite compound, and about twenty-three 

 hundredths Toy weight of ordinary air, which is a mechanical, and 

 slightly varying mixture, of oxygen, nitrogen, together with 

 minute quantities of all sorts of things capable of existing in the 

 gaseous state, and which assume that condition through the 

 natural operations that take place on the globe. Of a given 

 volume of air, oxygen constitutes about one-fifth, the remainiug 

 four-fifths being nearly all nitrogen, as the other substances, 

 ammonia, etc., found in air, only exist in minute propor- 

 tion. Oxygen is a very active body. Its name announces it 

 as an " acid-maker," which it is abundantly, though not exclu- 

 sively, as was once thought. It is what is called a great " sup- 

 porter of combustion," though not the only one; and ordinanj 

 cases of burning, whether fast as in a fire, or slow as in respi* 



