HARDWICKE'S SCIENCE-GOSSIP. 



171 



make notches with a " three square " file on opposite 

 sides of the glass where the cut is required. These 

 direct the wire and give a start, besides preventing 

 sideward cracking ; I have thus succeeded with the 

 string. 



Professor Langley's lecture on sunlight and the 

 earth's atmosphere, delivered at the Royal Institution, 

 is very interesting, as it contains a summary of his 

 researches on Mount Whitney, where he attained a 

 sufficient elevation to leave nearly half of the atmo- 

 sphere below him, and thus was able, with the aid of 

 instruments of especial delicacy, to compare the solar 

 radiations received up there with those which we 

 ordinarily receive down here at the bottom of the 

 atmospheric ocean. One of his broadest results is 

 the conclusion, that "the total loss by absorption 

 from atmosphere is nearly double what has been 

 heretofore supposed." Therefore the sum total of 

 the solar energy must be proportionally greater than 

 the usual estimate. He sets it down as capable of 

 melting a shell of ice sixty yards thick annually over 

 the whole earth, or "of exerting over one horse 

 power for each square yard of the normally exposed 

 surface." 



There is one inference stated in the report of the 

 lecture which puzzles me, viz., that " if the planet 

 were allowed to radiate freely into space without any 

 protecting veil, its sunlit surface would probably fall, 

 even in the tropics, below the temperature of freezing 

 mercury." 



In this there is a physical fallacy which I would 

 fain believe it impossible for Professor Langley to 

 perpetrate. In the case supposed there are two 

 bodies, the sun and the planet, opposite each other, 

 mutually radiating and receiving radiations. 



According to the well-established " law r of ex- 

 changes," when bodies are thus exposed to each 

 other and their temperatures are unequal, " the hotter 

 bodies will emit more radiations than they receive 

 from the colder bodies, and therefore, on the whole, 

 heat will be lost by the hotter and gained by the 

 colder till thermal equilibrium is attained." (J. 

 Clerk Maxwell.) This assumes that between the 

 bodies there is no absorbing medium, i.e. free space. 



It is clearly evident that under these circumstances 

 the cooler body, i.e. the planet (or say the moon, 

 which is an unprotected planet with one side thus 

 exposed), must be radiating less heat than it is re- 

 ceiving and therefore becoming warmer, and that the 

 temperature of " its sunlit surface " must be greater 

 without the protecting veil than with it. This con- 

 clusion is easily confirmed by experiment. A black 

 bulbed thermometer rises higher and higher when 

 exposed to direct solar radiations at greater and 

 greater elevations in a given latitude. Water may be 

 boiled on the snow fields of the higher Alps, by 

 simply placing it in a blackened copper vessel in a 

 blackened box with glass cover, and freely exposing 

 it to the solar radiations. 



My ' conclusion is that Professor Langley did not 

 mean what the reported words express. His meaning 

 must have been, not that its actually sunlit surface 

 would thus fall, but that a surface which had bee?t 

 sunlit, and is now dark, would fall ; not that the 

 bright side of the moon would fall below the freezing- 

 point of mercury, but that the dark side, or the side 

 that had been bright, would radiate away its heat as 

 rapidly as it received it. I have discussed this thus 

 fully, finding that Langley has been credited with 

 having proved, by his experiments, that the bright 

 surface of the full moon, in spite of the direct solar 

 glare, is colder than freezing water. 



His experiments show, that all our estimates of the 

 temperature of the lunar surface, based on comparison 

 with that of the earth, must be raised in proportion 

 to his correction of the amount of our loss by atmos- 

 pheric absorption. 



Our pre-eminence as " the land of tin " is 

 becoming seriously disputed. Cassiterite containing 

 94*895 per cent, of tin oxide is now found at Irish 

 Creek, Rockbridge County, Virginia, in loose 

 crystals, as fragments on the surface, and in veins. 

 The veins occur in a coarse grained, much decom- 

 posed, granite or gneiss. Besides the tin oxide, it 

 contains 3*418 per cent, of sesquioxide of iron, 0*760 

 of silica, 0*244 of lime, 0*27 of magnesia, and 0*237 

 of tungsten. If the commercial quantities correspond 

 with its chemical richness, this mineral will exert a 

 considerable influence on the metallurgical industry 

 of the United States. 



W. Hempel has made some experiments on the 

 combination of the different forms of carbon with iron, 

 with results that must be very disgusting to certain 

 superlatively practical people. Different parts of the 

 same piece of iron foil were equally exposed at a high 

 temperature to carbon in the form of diamond dust, 

 to graphite, and burnt sugar carbon. The diamond 

 dust did the work of converting the iron into steel, 

 while the graphite and amorphous carbon were 

 ineffectual. The heat was continued for two hours. 

 Ordinary cementation occupies about two weeks. 

 Other experiments have shown that carbon, in the 

 form of diamond, combines with iron at a lower 

 temperature than either of the other forms of carbon. 



Within the reach of my own recollection, as a 

 teacher of chemistry, the silvering of glass by pre- 

 cipitation of actual silver was merely a laboratory or 

 lecture-table experiment. Now it is extensively 

 used for the practical manufacture of mirrors on a 

 large scale, superseding the old amalgam of mercury 

 and tin foil. It has done good service to the 

 astronomer by supplying him with "silver-on-glass " 

 mirrors for reflecting telescopes, which are now so 

 extensively superseding the more costly and ponder- 

 ous speculum metal. Bottger, in a recent paper, 

 recommends the following proportions of materials to 

 be used. Dissolve four parts of pulverised nitrate 

 of silver in strong ammonia. Then add to this one 



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