172 ATTRACTION. 



be convenient to use it occasionally, but it will be understood by the 

 reader, that nothing more is intended by it than combining weight, 

 combining proportion, or chemical equivalent.* 



It will doubtless be thought by some, that the atomic theory should 

 be presented more in detail. There can be no objection to its be- 

 ing studied fully by those who are well versed in chemistry, but the 

 learners of elements, for whom chiefly this work is intended, will, if 

 they have mastered the doctrine of definite and multiple proportions, 

 be able to go forward in their studies without the atomic theory, and 

 to understand that theory the better, the farther they proceed in the 

 science. We do not, however, hold it in small consideration, and a 

 sufficient number of opportunities of illustrating its nature, will pre- 

 sent themselves in the study of the particular bodies. f 



APPENDIX TO ATTRACTION. 



Terrestrial and artificial magnetism, has an evident effect on chem- 

 ical action. Before leaving the subject of attraction, it ought to be 

 remembered, that magnetism appears to be connected with it. Tinc- 

 ture of purple cabbage placed in a syphon tube, is changed in fifteen 

 minutes to green, by being connected by an iron wire, with the two 

 poles of a magnet, and when the liquor was in two connected tubes, 

 the same thing happened, but it required two days to effect the 

 change. J 



A' syphon tube, half an inch wide, and four and five inches long, 

 having mercury poured into the bend, but not sufficient to cut off the 

 communication between the two branches ; the tube is then nearly 

 filled wijth an acid solution of nitrate of silver. The tube being placed 

 in the plane of tlie magnetic meridian, the precipitation of the arbor 

 dianse is much more rapid than when it is at right angles with it; and it 

 is much more abundant at the north than at the south end, and the 

 crystals are more brilliant and longer, and more perfect. 



A bent tube placed across the magnetic meridian, and in which 

 the crystallization has made little progress, exhibits it in increased 

 activity, when two artificial magnets are approached, the north pole 



*Dh "VVollaston, in a paper on the finite extent of the atmosphere, published in 

 the Phil. Transactions for 1822, has rendered it probable that there are atmospheri- 

 cal atoms incapable of farther division. The question as to the indivisibility of 

 atoms, is a physical topic, entirely independent of the mathematical speculation as to 

 the infinite divisibility of matter ; a speculation which seems however to have little 

 utility, and some would say, meaning, except with reference to physical elements. 



t Thenard has followed this course, Vol. I, Chem. p. 24, Ed. 5. I heard Mr. 

 Dalton explain his own theory in his lecture room at Manchester, and while I 

 was entertained with the arrangement of his atomic symbols, I was forcibly struck 

 with the stiil greater value of his discovery of multiple proportions. 



i The spontaneous change is to red and not to green. 



A fanciful name given to this peculiar crystallization of silver; the disposition 

 of the crystals being in branches, and silver was formerly called Luna or Diana. 



