Henry's Chemistry. 341 



" in various cases, when two acids are brought into contact with 

 one base, the base unites with one acid, to the entire exclusion of 

 the other." Dr. Henry might have also adduced the mutual pre- 

 cipitation of metals from their saline combinations, in which the 

 displacement is complete, and not partial, as Berthollet's doctrines 

 lead us to infer, when the force of cohesion is equally active. Yet 

 Dr. Henry, in his eighth section, entitled Experimental Illustrations 

 of Chcinical Affinity, seems to lose sight of what he had previously 

 propounded ; for he prints the following axiom in italics as one to 

 be illustrated by experiment. " XIV. In every instance, in 

 comparing the affinities of two bodies for a third, a weaker affinity 

 in one of the two compared will be found to be compensated by 

 increasing its quantity." We should like to know what quantity of 

 silver or of oxalic acid may be requisite to decompose an ounce of 

 sulphate of lead. 



Dr. Henry's third chapter is dedicated to Caloric. The facts, 

 which are distributed into four sections, appear to be well selected, 

 and fairly stated. In describing M. Breguet's thermometer, in the 

 second section, he appears to misunderstand its construction. " It 

 consists," says he, " of a slip of silver, and another of platinum, 

 coiled into a spiral, one end of which is fixed, while the other is 

 connected with an index, which traverses a graduated circular 

 plate." There is only one slip, in which the platina and silver are 

 united face to face with gold solder. The difference of expansion 

 between the two metals by variation of temperature, causes the 

 spiral to increase or diminish the degree of its curvature, and thus 

 makes the needle traverse the graduated circle, attached at right 

 angles to the axis of the spiral. 



The 4th Chapter treats of light, the phenomena of the prismatic 

 spectrum, solar phosphori, &c. ; and in the 5th we have brought 

 before us the important subject of " the chemical agencies of com- 

 mon and galvanic electricity," a title which would have pleased us 

 better, had the words " common and galvanic" been omitted ; 

 but this is a trifle, and the chapter itself a very good one ; it 

 treats in successive sections of the construction of galvanic appa- 

 ratus, of the identity of galvanism and common electricity; of the 

 chemical agencies of electricity ; of the theory by which they are 

 best explained ; of the hypothesis of the origin of electricity in 

 galvanic arrangements, and of the phenomena of electro-magnetic 

 motion : upon a few of the topics discussed in this chapter, we 

 must beg leave to offer a few observations. 



And in the first place, we should advise our author in his next 

 edition, entirely to re-modcl the first section, in which we are 

 brought at once to the discussion of simple galvanic circles, and 

 compound galvanic circles, without a word of prefatory matter 

 respecting the laws of electrical excitation, and the new properties 

 which bodies acquire whilst under its influence ; we should also 



