336 Analysis of Scientific Boohs. 



cohesion, solution, and crystallization. " The cohesive affinity," 

 says Dr. Henry, " is a property which is common to a great variety 

 of bodies. It is most strongly exerted in solids, and in them it is 

 proportionate to the mechanical force required for effecting their 

 disunion. In liquids, it acts with considerably less energy ; and 

 in aeriform bodies, we have no evidence that it exists at all ; for 

 their particles, as will afterwards be shewn, are mutually repulsive, 

 and if not held together by pressure, would probably separate to 

 immeasurable distances. Water also in a solid state has con- 

 siderable cohesion, which is much diminished when it becomes 

 liquid, and is entirely destroyed when it is changed into vapour." 



We have quoted this passage, not so much for the purpose of 

 exemplifying the practice too common with some systematists of 

 rilling up their pages with trite and unmeaning generalities, as to 

 point out its false philosophy. Fully a year before Dr. Henry's 

 work appeared, Dr. Wollaston investigated the constitution 

 of elastic fluidity in a manner worthy of his genius, and had 

 shewn, that " all the phenomena accord entirely with the sup- 

 position that the earth's atmosphere is of a finite extent, limited by 

 the weight of ultimate atoms of definite magnitude, no lunger divi- 

 sible by repuhion of their •parts*.'" It hence appears that air, not 

 " held together by pressure, would {not) probably separate to im- 

 measurable distances." 



" Cohesive affinity," says Dr. Henry," is in solids proportionate 

 to the mechanical force required for effecting their disunion." He 

 should here have distinguished between hardness and tenacity. 

 He should likewise have bestowed at least a wooden cut or two on 

 Haiiy's theory of crystallization; for want of which his account 

 of the matter must merely perplex the student. The term refecting, 

 however appropriate to the inventor of the goniometer, is not the 

 correct one for the instrument itself, which was originally named 

 reflective. 



In section 3, of the chapter on Chemical Affinity, Dr. Henry 

 treats of the proportions in which bodies combine, and of the 

 atomic theory t. 



* On the finite extent of the atmosphere. Read at the Royal Societv. 

 January, 17, 1822. 



t As persons, otherwise clear-headed and well-informed, are continually ex- 

 pressing their inability to comprehend this part of chemistry, we shall en- 

 deavour to divest it of rigmarol, and to shew them that it is a branch of chemi- 

 cal theory founded upon a very tew and simple facts, which' when once in their 

 possession, will enable them not merely to understand its principles, but to 

 appreciate its usefulness and importance. If we subject any true chemical 

 compound to analysis, vie shall rind that its elements are always in the same 

 proportions, and that from whatever source it be derived, or under whatever 

 circumstances it be formed, they uniform!}' exhibit the same relative propor- 

 tion to each other. Water, for instance, is a compound ot hydrogen and oxy- 

 gen, in which the former always bears to the latter, the proportion of 1 to 8. 

 In dry sulphate of magnesia or Epsom salt, the sulphuric acid is to the mag- 

 nesia uniformly as 40 to 20. Pure white marble, whether it comes from Paros 



