M 



UNDULATORY FORCES. HEAT. 



[on. TYNDALL'S DISCOVERIES. 



Elasticity of Vapour of Water (continued). 

 Temperature. Inchet of Mercury. 



80 Ure 1.01 



H 

 00 

 100 

 110 

 120 

 ISO 

 130 

 140 

 150 

 150 

 160 

 160 

 170 

 170 

 180 

 190 

 200 

 212 



Dalton 

 Both 



Dalton 



1.00 

 1.36 

 1.86 

 2.63 



4.34 

 4.36 

 5.77 

 7.53 



Dalton 7-42 

 9.46 

 9.60 

 12.05 

 Dalton 12.13 

 Both 15.15 

 19.00 

 23.60 

 30.00 



Ure 



Uro 



Boiling Point of various Liquids. 



Sulpluirio ether, of an average specific gravity 96" F. 

 Alcohol, of an average specific gravity . 1 74 



Water 212 



Saturated solution of common salt . .218 

 Nitric acid . . . . . . .242 



Oil of turpentine 600 



Sulphuric acid 580 



,, of lower specific gravity . 550 



Linseed oil ..... . GOO 



Mercury Dalton GUO 



Irvine 073 



Melting Point* of various Substances. 



Sulphuric Ether . . . . 47 F. 



Vcrcury 38 



Vinegar, and Milk . . . + 28 



Ice ...... 33 



Phosphorus 100 



Potassium 136 



Alloy of Cadmium, 3 parts > 



-l m 



Lead . 8 



,, ,, Bismuth 15 



Sodium ..... 194 

 Alloy of Tin . . 3 parts i 



Lead . 6 . .210 



,, Bismuth 8 



Sulphur 22G 



Tin 442 



Bismuth 476 



i. 493 



Lead 694 



. .... 630 



Zinc 700 



Brass (with 25 per cent, of zinc) . 1750 



Brass (ordinary) .... 1869 



Silver 1280 



, 1873 



Copper .... Daniell 1996 



2548 



Gold .... Daniell 2016 



2590 



Cast Iron 1780 



. . . . . 3479 



Platina, and metals associated with it, are infusible 

 except undor the flame of the oxy -hydrogen blow-pipe. 



ADDENDA ON RADIANT HEAT. 



WHILST the previous pages were passing through the 

 press, some highly interesting observations and experi- 

 ments, by Dr. Tyndall, F.R.S., of the Royal Institu- 

 tion, have been communicated to the scientific public ; a 

 summary of which we append to this section. 



Referring the reader to our remarks on Radiant and 

 Terrestrial Heat,* he will find, that we explained, under 

 those heads, the general laws of Radiant Heat ; and 

 also alluded to some of their applications in reference 

 to many natural phenomena. We found that various 

 substances had different powers of absorbing and radiating 

 caloric. In another portion of our pages, t we stated, 

 that some gases, liquids, and solids, prevent the passage 

 of calorific rays more than others ; and, also, that heat, 

 radiating from any source, undergoes at tunes both re- 

 fraction and polarisation. Our remarks also extended 

 to the "diathermancy, "or general power of various bodies, 

 in permitting the transmission of calorific rays through 

 their substance at unequal speed. 



Dr. Tyndall has extended our knowledge of these 

 various facts, more particularly in reference to the radi- 

 ating power of the earth, and the absorbing or preventing 

 influence which the atmosphere, resting on its surface, 

 exerts. By numerous experiments on vapours, with 

 respect to the amount of rays of heat of which they per- 

 mit transmission, we learn from him, that non-luminous 

 rayg pass most completely through dry air ; whilst some 

 gases and vapours entirely stop their progress, such as 

 olefiant gas and the vapour of other. 



Carbonic acid is found to a limited extent in our 

 atmosphere; and Dr. Tyndall states, that this, in its 

 gaseous form, permits of the rapid transmission of radiant 

 and non-luminous heat, and the consequent cooling of 

 the radiating surface of the earth. An antagonism to 

 this result is found not only in the dense clouds so com- 

 mon in our climate, but also in the action of aqueous 



See ante, pp. SO, 30. t See ante, p..28; " Refraction." 



vapour, which is always present in the air in temperate 

 climes, and prevents the radiation of non-luminous heat. 



Viewing these facts in their practical results, we can- 

 not help being impressed with a deep sense of their 

 value. Under a clear sky, and in dry air, as observed so 

 frequently in the Arctic regions,' intense cold continually 

 prevails, and, at the same time, the atmospheric tempera- 

 ture is such as not to allow of the presence of aqueous 

 vapour. As, however, we approach the tropics, the 

 average temperature of the air increases, and with it the 

 amount of aqueous vapour existing in it. Now this, as wo 

 have already mentioned, has been pointed out by Dr. 

 Tyndall as acting after the manner of an invisible screen, 

 and preventing the great loss of terrestrial heat which 

 would occur through its absence, in the ordinary process 

 of terrestrial radiation. The general application to Me- 

 teorology, of these principles, has yet to be made ; but we 

 can easily perceive, that the production or modification 

 of an immense variety of atmospheric phenomena may 

 result from the action of these laws. 



Having thus given a general idea of the facts which 

 Dr. Tyndall has developed, wo quote from the Saturday 

 Review, of March 2nd, 1801, a more detailed account of 

 his experiments, as the best resume to which we have 

 had access ; and which will afford our readers considerable 

 information on the subject. 



" With his admirable thermo-electric apparatus, 

 Melloni was unable to detect any trace either of the 

 absorption or radiation of heat by air ; and we possess 

 no knowledge regarding the action of other gases or 

 of vapours. In the Bakerian lecture, recently delivered 

 before the Royal Society, by Professor Tyndall, the fol- 

 lowing new results in connexion with this subject wore 

 enunciated. 



" Oxygen, hydrogen, nitrogen, and air, enclosed in a 

 tube four feet long, absorb respectively about three- 

 tenths per cent, of the calorific rays emanating from a 



