202 Prof. Kelland's Reply to some Objections against the 



These crystals, submitted to measurement by Professor 

 Miller, were obtained by slow spontaneous evaporation: the 

 difficulty of procuring this salt in crystals of any size has been 

 fully pointed out by Mr. Croft. 



I have only one observation which does not coincide with Mr. 

 Croft's statements, but which, however, confirms in a great 

 measure the results of his analysis; I allude to the statement 

 that these double salts of chromium cannot be formed by the 

 direct combination of their ingredients. The process which I 

 have followed has been to digest the hydrated oxide of chro- 

 mium in a mixed solution of oxalic acid and oxalate of potash 

 in the proportions indicated by analysis, and when it ceases to 

 dissolve the oxide, to decant the clear solution and allow it to 

 crystallize. By the same means the analogous salts of soda 

 and ammonia have been obtained, but not in crystals suffi- 

 ciently large for measurement, as also other double salts of 

 chromium. To prepare the hydrated oxide of chromium, 

 the best and most ceconomical process that I have found, is 

 to take 150 grs. of the bichromate of potash and 200 grs. of 

 liquid sulphuric acid, oil of vitriol, these proportions being 

 nearly in the ratio of their atomic weights, so that the chrome 

 alum, sulphate of the green oxide of chromium and potash, 

 may be formed ; the deoxidation of the chromic acid is easily 

 effected by the addition of a little sugar and boiling the solu- 

 tion. When the deoxidation is complete, the green oxide 

 may be precipitated by ammonia or by a carbonated alkali, 

 and only requires to be well washed to remove all trace of 

 alkali or saline matter. 



XXXVI. Reply to some Objections against the Theory of Mo- 

 lecular Action according to Newton's Lww. By the Rev. P. 

 Kelland, M.A., F.R.SS. L. $ E., F.C.P.S., &>c, Professor 

 of Mathematics in the University of Edinburgh, late Fellow 

 and Tutor of Queen's College, Cambridge. 

 [Continued from p. 130.] 

 2. HPHE next objection to the molecular hypothesis of par- 

 -*• tides acting on each other, with forces varying inversely 

 as the square of the distance, is that the equilibrium of such a 

 system would not be stable. This objection is stated by Mr. 

 Earnshawin his memoir, Art. 15. The argument is as follows. 

 The force due to a displacement parallel to either principal 

 axis depends on the second differential coefficient of V, with 

 respect to the coordinate along that axis. Now the sum of 

 the second differential coefficients for the three coordinates is 

 zero. Hence one of them must be positive, and the corre- 

 sponding force put in play acts to draw the particle^owz its 

 system of rest. Of course this reasoning depends on the as- 



