June 22, 1900.] 



SCIENCE. 



987 



introduction of Sutherland's use of molecular 

 attractions and by a more detailed eifort of 

 the author to justify, from experimental re- 

 sults, his own contention that the coefBcient 

 of diffusion of two gases is variable with the 

 proportions of the mixture. ' ' D therefore 

 assumes a different value at evei'y different 

 place in the mixture that is being formed 

 by the diffusion" (p. 262). He admits that his 

 own formula for D makes the calculation of 

 diffusion ' excessively laborious,' and is chari- 

 table to those who have preferred to use a more 

 convenient though, as he maintains, inaccurate 

 one. 



Chaptee IX. — Conduction of Heat. 



In his first edition the author derives from 

 theoretical considerations the formula 



A; = 1.530w, 



where h is the thermal conductivity, tj the co- 

 efiicient of viscosity, and c the specific heat at 

 constant volume. He now finds 1.6027 instead 

 of 1.530. Other changes are indicated by the 

 following quotations : "In fact, the assumption 

 has many times been made, especially by Stefan 

 and Boltzmann, that the kinetic energy of the 

 molecular motion is passed on from place to 

 place with greater speed than the remaining 

 energy, which in Chapter V. we have tei'med 

 the atomic energy. We were at that time 

 obliged to yield to this view, because no other 

 possibility was seen of bringing the theoretical 

 law k = Kiio into a complete agreement with the 

 observations then published" (p. 284). But 

 now, "The excellent agreement of the calculated 

 and observed values ". * * * "justifies in us, 

 on the contrary, the conviction that the accu- 

 racy of the theoretically deduced relation be- 

 tween the conductivity and viscosity of gases 

 is no longer to be doubted, and that we may 

 take it as proved that a gas has the same con- 

 ductivity for every kind of energy. From this 

 result of theory we see finally that viscosity, 

 diffusion, and conduction of gases depend in the 

 same way on the free path of the gaseous par- 

 ticles, and that each of these three phenomena 

 may be employed to determine the value of the 

 molecular free path " (p. 296). 



Chapter X. — Direct Properties of Molecules. 



§§ 111 and 112 of this chapter carry further 

 than the old edition does the argument in 

 proof of the general flatness of molecules. 

 " These examples seem to indicate that the sec- 

 tion of a molecule is equal to the sum of the 

 sections of the atoms which form it" (p. 302). 

 " If the hypothesis were general and exact that 

 the section of the molecule of a chemical 

 compound is equal to the sum of the sec- 

 tions of its atoms, it would allow of but 

 a single interpretation, and thereby permit an 

 interesting peep into the circumstances of ar- 

 rangement of the atoms. We should not be at 

 liberty to make any other assumption than that 

 the atoms which are bound together into one 

 molecule are all in one plane" (p. 304). 

 " When four atoms are joined together to form 

 a molecule it is in general no longer necessary 

 for them to possess the property of being a plane 

 system ; the possibility, however, of the system 

 being of such a character is shown by the ex- 

 ample of ammonia." * * * "We shall con- 

 sequently be unable to make any other sup- 

 position as to the molecular constitution of 

 ammonia than that usual with chemists, viz, 

 that the three atoms of hydrogen are so arranged 

 that their common centroid is always within 

 the atom of nitrogen, and that they circle about 

 this atom in plane orbits" (p. 305). "And so 

 good an agreement is exhibited by the great 

 majority of the values at 0° for gases and va- 

 pors that we have to conclude in general that 

 their molecules have a shape that is flat, and not 

 spread out on all sides into space. This view 

 seems to be the most probable, at least for the 

 gaseous state " (p. 309). 



The last paragraph of § 116 reads thus : 

 "From these considerations we can conclude 

 only that the gaseous molecules are smaller than 

 a sphere whose diameter is one-millionth of a 

 millimetre. But we may add as very probable 

 that the size of the gaseous molecules will in no 

 way appear to be vanishingly small when com- 

 pared with that small sphere. This is justified 

 on many other grounds, which we have still to 

 mention." 



§ 118, on calculation of the size of the mole- 

 cules from the dielectic capacity, is new. " The 

 molecules of such substances [dielectrics] are 



