SOME HARDENING AGENTS. 381 



ride will be dotted with small black specks or star-like crystals of 

 the salt. They should be washed directly in various strengths of 

 alcohol. In the picric acid tissues, water swells and injures the 

 specimens. 



i6. — Use several hardening agents on one tissue and test the 

 results. No definite instructions can be given for individual cases, 

 but the following general rules will assist in determining what 

 should be the nature of the hardening fluid : — 



(a) Where a tissue is hard and firm, and not likely to shrivel on 

 the abstraction of water, and where, too, it is not thought neces- 

 sary to keep blood in the organ, spirit may be used. 



(d) Where there is much blood in the tissue to be hardened, 

 or where the tissue is very soft or oedematous, use Miiller. 



(c) For small objects of very delicate structure, use osmic acid, 

 Rabl's, Flemming's, Kleinenberg's, chromic acid, or Miiller. 



(d) If bacilli or bacteria are suspected, use alcohol, weak or 

 absolute. In some cases, however, a previous treatment with 

 Miiller's fluid is a great advantage. 



If these directions are carefully followed, good results will be 

 obtained, and for successful pathological and histological investi- 

 gation so much depends on this preliminary work — (in spite of its 

 being said, " We have such perfect methods for embedding it is 

 not necessary") — that I advise all to pay every attention to them, 

 even to the minutest detail. 



Origin of Atmospheric Oxygen. — Dr. T. i. Phipson, who 

 has devoted a considerable amount of attention to problems con- 

 cerning the constitution of the atmosphere, is led to the conclusion 

 that the original atmosphere of the globe consisted of nitrogen 

 alone, and that the oxygen now present is the product of vegetable 

 life. In a paper in the Chemical News^ he states that minute 

 microscopic plants {Frotococais phwialis Tind P. palustris) can be 

 easily transformed into manufacturers of oxygen gas. As the 

 results of experimients, he concludes that plants absorb carbonic 

 acid gas by the roots and secrete oxygen by the leaves, from which 

 it is subsequently given off. Into the primitive atmosphere of 

 nitrogen the early vegetation would thus pour oxygen during 

 countless years, until its composition became practically what it is 

 now. — Pharmaceutical Journal. 



