212 ANNUAL OF SCIENTIFIC DISCOVERY. 



AN rMPROVED QUALITATFTE FILTER. 



A mode of folding filters for qualitative n,nal3'sis superior to the 

 one in common use is suggested by C. E. Avery. In place of 

 folding the filter doubled upon itself down the middle in the 

 usual way, he turns down on each side of the paper a fold equal 

 to one-quarter of the semicircle, and then folds the sectors of 45° 

 thus formed back upon themselves. The filter is then opened 

 without disturbing the folded portion, and phiced in the funnel. 

 In this form the triple side of the plain filter is broken up, and 

 the folded portions keep open passages instead of hindering 

 filtration. A gain of some 50 per cent, is obtained in the rapidity 

 of filti'ation. — Amer. Jour. Phar.^ XL., p. 200. 



THE PHENOMENA OF STTPERSATURATION AND BOILING. 



Oersted noticed in 1806 that dilute acids might be cautiously 

 added to solutions of the alkaloid carbonates without producing 

 any action, but that on the introduction of a solid body, such as a 

 platinum wire, a glass rod, or the finger, brisk eflfervescence en- 

 sued, lie inferred that gas in solution is never given off, except 

 in contact with a solid, and he adduced the influence of solids in 

 promoting crystallization in support of his vicAV. Analogous ex- 

 periments were made by Schoubcin in 1837, who suggested that 

 the solids acted by carrying down air. Further illustrations were 

 supplied, in 1839, by Liebig, who concurred in attributing the ef- 

 fects to the influence of the air introduced. Tomlinson refers all 

 such phenomena, together with that of the sudden deposition of 

 ciystals from a supersaturated solution, to the action of nuclciy and 

 defines a nucleus as " a bodv which has a stronger adhesion for a 

 gas, salt, or vapor in solution than for the liquid which holds 

 it in solution." The action of solids in these cases he ascribes 

 to the greasy film which after exposure to the air they are sure to 

 acquire ; for it is found that a platinum or a glass rod, which, by 

 means of chemical agents, is made perfectly clean, is powerless 

 to bring about such changes. He considers the sudden cr3'stalliza- 

 tion of a supersaturated solution of a salt by mere exposure to the 

 air as due to particles of dust falling upon the liquid. 



The efl'^t of drojDping a crystal of the salt into the solution is 

 due to its having acquired the condition of a nucleus from expo- 

 sure to the air. If a cr3stal of sulphate of sodium be suspended 

 in the neck of the flask in which the supersaturated solution is be- 

 ing prepared, it ceases to be a nucleus, and when the solution is 

 cold the ciystal may be lowered into it without determining the 

 separation of the salt. Tomlinson uses tlie term caiharism to 

 express the condition of a body which is made chemically clean, 

 and wliich does not act as a nucleus. Considering a boiling liquid 

 as a supersaturated solution of its own vapor, he refers to the same 

 theory the phenomena of boiling. If any solid body be intro- 

 duced into a boiling liquid, bubbles of vapor collect on the solid 



