PHYSICS. 241 



even under inches water-pressure. The depth-gauge is made by tying 

 a piece of cloth over the mouth of the larger leg of a U-tube, the end of 

 the smaller being closed by a stopper. On sinking it the air is com- 

 pressed, and water enters through the cloth into the smaller leg. On 

 raising it the air expands and forces the water out through the cloth to 

 the last drop. The depth can be determined from the water left in the 

 smaller leg of the instrument. — {Nature, xxii, 548, 1880.) 



3. Of Gases. 



Hannay and nogarth have communicated to the Eoyal Society a re- 

 markable iiaper on the solubility of solids in gases, the research being 

 the starting point of the experiments which have since enabled the 

 former to i)roduce crystallized carbon. They desired to investigate the 

 solvent power of liquids at temperatures at or above their critical points. 

 For if at the critical point there really occurs a transition from the liquid 

 to the gaseous state, and if the property of dissolving solids is one pos- 

 sessed by liquids alone, there ought to be precipitation of the dissolved 

 solid matter as the substance passes through the critical point. Absence 

 of such precipitation would furnish a strong independent proof of the 

 perfect continuity of the liquid and gaseous states. As a preliminary 

 test, a solution of potassium iodide in alcohol was prepared and a strong 

 tube one half filled with it. No precipitation of any solid was observable 

 even at 350^" 0., more than 100° above the critical point of alcohol. 

 Eesin dissolved in paraffin spirit showed no decomposition at 300°. 

 With an improved and simplified apparatus, it was shown that a clean 

 crystal of potassium iodide dissolved entirely in alcohol gas (the term 

 " gas" referring to the tiuid above its critical point). Potassium bromide 

 and calcium chloride were also soluble in this gas. Oobaltous chloride 

 remained soluble at 320° and showed its blue color and characteristic 

 spectrum. Even the acid decomposition-product of chlorophyll, which 

 decomposes in the air below 200°, gives, when dissolved in alcohol, iden- 

 tical spectra at 350<^ and at 15° 0. Some curious reactions were ob- 

 served. Thus alcohol or ether deposits ferric chloride just below the- 

 critical point, but redissolves it in the gas when raised 8° or 10° above 

 that point. By suddenly reducing the pressure, the solid is precipitated 

 crystalline, falling as a snow. It is redissolved in the gas as the i)ress- 

 sure increases. — {Proc. Roy. Soc, xxx, 178 5 Nature, xxi, 82, 499, 1880.) 



Bottomley has described a neat experiment by Quincke, first sug- 

 gested by some experiments to test the porosity of glass. A glass tube, 

 5 inches long and thi'ee-fourths inch external diameter, witli stout walls, 

 was closed at one end and constricted a little more than half way up. 

 Into the lower closed portion some sulphuric acid was poured, and above 

 the constriction some fragments of marble were placed. The tube was 

 then drawn off at top, thickened, and carefully annealed. On inverting 

 it the acid attacked the marble, and carbon dioxide was evolved. For 

 the first few days nothing peculiar was noticeable; but soon the liquid 

 S. Mis. 31 16 



