On Crystalline Forms and Chemical Formnlce. 405 



the spiral is weak only, and the part even which the magneto- 

 electric current plays in the process of combustion is also 

 slight. This part is, with regard to the opposed directions, 

 at the contact, of the galvanic and of the magneto-electric 

 current, even possibly negative. However, it may be difficult 

 to demonstrate by direct observation the modification which 

 connects the process of combustion, at the contact, when 

 an electro-magnet is situated in the circle of the circuit, pre- 

 supposing, that is, that the power of the galvanic current is in 

 both cases the same, which equality must be produced by an 

 increase in some of the electromotors corresponding to the in- 

 duced opposition of conduction of the spirals. 



LIV. On some apparent Exceptions to the Laiso^ that liJce Cry~ 

 stalline Forms indicate like Chemical Formula. By James 

 F. W. Johnston, M.A., F.R.SS. L.^E., F.G.S., Sfc. Sfc. 

 Prof. Chem. and Min., University, Durham.* 



"C^OLLOWING up the subject of a former paper on a sup- 

 -■- posed analogy in atomic constitution between the earthy 

 carbonates and the alkaline nitrates f , I proceed to examine 

 certain other cases of isomorphous coincidence between sub- 

 stances of which,the chemical formulae according to received 

 views are wholly irreconcileable. 



The first exception to the law that substances which re- 

 place each other may be represented by analogous formulae 

 was deduced by Mitscherlich from the observation that am- 

 monia with an atom of water (NH3+HO) may replace pot- 

 ash (KO) in its salts, without changing their crystalline form. 

 Six years have since elapsed and we are still without conclu- 

 sive proof of any other mutual replacement of substances re- 

 presented by unlike formulae. There are indeed one or two 

 cases in which appearances seem to render such replacement 

 prohahlc, but as yet no other case has been established. 



We are acquainted however with a considerable number of 

 substances, which though unlike in constitution, and incapable, 

 or not known to be capable, of replacing each other, yet cry- 

 stallize in forms which are almost or altogether identical. 

 In the following table, extracted from my Report on Dimor- 

 phous bodies %, I have arranged all the cases which have hi- 

 therto come to my knowledge. 



* Communicated by the Author. f See this Journal, xii. p. 480. 



X Report of the British Association, vol. vi. p. 175. In page 209 of the 

 Report on Dimorphous bodies, line 5, for Isomorphwn re?id Jsomerisvi. 



