ATMOSPHERE. 



51 



Atmo- 

 sphere. 



Specific 

 gravity 



fair. 



Expansion 

 by heat. 



guished themselves by their investigations of the at- 

 mosphere, a list which would include almost all the 

 celebrated names of the last century. 



From the experiments of Sir George Shuck- 

 burgh Evelyn, (Phil. Train. 1777 and 179S,) made 

 with a degree of precision and patient industry, which 

 perhaps have never been surpassed, it appears, that 

 at the temperature of 60, when the barometer stands 

 at 30 inches, the specific gravity of atmospherical air 

 is 0.001208, that of water being 1.000, or its weight 

 is to that of water as 1 to 828. Hence 100 cubic inches 

 of it under that pressure and at that temperature weigh 

 30.5 grains : For a cubic inch of pure waterat that tem- 

 perature weighs 252.506 grains, according to experi- 

 ments of Shuckburgh corrected by Mr Fletcher. 

 (Nicholson's./fw>rfl/,iv. 35.) The resultof the expe- 

 riments of Lefevre Gineau, who was employed by the 

 French government to ascertain the weight of water, 

 in order to fix their standard of weights, was some- 

 what different. According to him, a cubic inch of 

 wateriat 60 weighs 252.72 grains troy. The dif- 

 ference may be partly owing to some small error in 

 the allowance of the expansion of water from 4-0, the 

 temperature at which his experiments were made, to 

 60. At any rate, the known precision, and the ex- 

 cellent apparatus of Sir George Shuckburgh, entitle 

 his result to the preferetwe. Hawksbee's experiments 

 make air 850 times lighter than water, the barometer 

 being at 29.7, and Dr Halley supposed it about 800. 

 But neither of t'tiese numbers is to be put in compe- 

 tition with the result of Sir George Shuckburgh 

 given above. The air when weighed, is supposed to 

 be in its usual state of dryness ; when very moist, its 

 specific gravity is diminished. An exact knowledge 

 of the weight of a given bulk of air is of gr^at im- 

 portance, because it enables us with much tacility to 

 ascertain the weight of all other aerial bodies : for it 

 is easy to determine the relative weight of any elastic 

 fluid to that of air. 



When heat is applied to atmospherical air, its 

 bulk increases ; while cold, on the other hand, dimi- 

 nishes its bulk. As this change in bulk is very con- 

 siderable, it affects very much the accuracy of all ex- 

 periments on it. It has therefore been an object with 

 philosophers to determine the precise amount and rate 

 of the change in bulk produced upon air by heat. 

 M. De Luc, Sir George Shuckburgh Evelyn, Ge- 

 neral Roy, Mr Dalton, and Mr Gay Lussac, are the 

 gentlemen to whom we are indebted for the solution 

 of this problem. In examining the dilatability of air 

 by heat, it is necessary that no water be in contact 

 with it. For as heat converts water into vapour, this 

 vapour mixing with the air, would destroy the accu- 

 racy of the results, and make the dilatation appear 

 much greater than it really is. According to the ex- 

 periments of De Luc, air at the temperature of 55 

 when heated 1 of Fahrenheit's thermometer, ex- 

 pands ^j-y-j- part ; according to Shuckburgh, the ex- 

 pansion is ^-j^- ; according to General Roy, it is ^-fr? 

 according to Dalton, it is ;r | T ; and according to Gay 

 Lussac, tyv' i ^ 8 Dalton and Gay Lussac were at 

 pains to exclude moisture, we may consider their ex- 

 periments as more accurate than those that preceded 

 them. As to the rate of expansion, General Roy 

 fouud it a slowly diminishing ratio from 32 to 212. 



Mr Dalton found the same thing. But he considers 

 this diminution as apparent only, and not real, and 

 owing to the expansion of mercury not being equable. 

 According to him, water, mercury, and all liquids, 

 expand as the square of the temperature, reckoning 

 from the freezing point of the respective liquid. Ac- 

 cording to this notion, the expansion of air, (and in- 

 deed of all permanently elastic fluids,) is in geome- 

 trical progression to equal increments of temperature. 

 The following Table exhibits the rate of expansion of 

 air from 32 3 to 212, according to Mr Dalton: 



Degrees of fi lfc . of AJr< Degrees of Dal- 



Fahrenheit. ton > 1 nermometcr. 



32 1000 32 



39.3 1017.9 42 



47 1036.1 ..... 52 



55 1054.7 62 



63.3 1073.5 72 



72 1092.7 ..... 82 



81 1112.3 92 



90.4- 1132.2 102 



100.1 1152.4 112 



110 1173.1 122 



120.1 1194 132 



130.4 1215.4 142 



141.1 1237.1 152 



152 1259.2 162 



163.2 1281.8 172 



175 1304.7 182 



Atmo 

 sphere. 



186.9 1328 



199.2 1351.! 



212 1376 



359.1 1643 



539.8 1962 



754.7 2342 



1000 2797 



1285 3339 



192 

 202 

 212 

 312 

 412 

 512 

 612 

 712 



The reader will observe, that the expansion of air in 

 the second column of the- Table constitutes a geome- 

 trical progression, the ratio of which is 1.0179. The 

 third column exhibits the corresponding degrees of a 

 Fahrenheit's thermometer graduated, according to Mr 

 Dalton's notion of the expansion of mercury, accord- 

 ing to the square of the temperature. This notion 

 of Dalton must be allowed to be very ingenious. 

 Unfortunately we are not in possession of any mode 

 of ascertaining how far it is correct. It is only sup- 

 ported by analogical reasoning, and cannot well be 

 otherwise in the present state of our knowledge of 

 heat. 



Atmospherical air was long considered as a sim- Compos- 

 pie elementary body. But it is now known to con- t,on f tne 

 sist of at least four distinct substances,namely,oxygen, atmos P nere 

 azote, carbonic acid, and aqueous vapour. The first 

 two substances must be considered as its essential con- 

 stituents, and constitute in fact almost the whole of 

 it. The other two are variable in their proportion, 

 and exist only in minute quantities, which it is diffi- 

 cult to appreciate. 



The first knowledge of the composition of the at- History of 

 mosphere must have been after the period of the dis- 'he discO- 

 covery of oxygen gas by Dr Priestley in 1774. La- very, 

 voisier, indeed, in his posthumous works, appears to 

 insinuate a knowledge of it in 1772. But this claim 

 cannot be admitted, as he gives no hint of any such 



