Dr. E. J. Mills's Researches on Melting-point. 29 



Such illustrations may be of service in enabling us to detect, 

 with more or less probability, the parallelism of chemical 

 series, and to enable us to decide whether a function — nitra- 

 tion, for example — has or has not the same value in different 

 parts of a series. Other groups in which a similar but less 

 intimate relation prevails, might be adduced from the list ; 

 and a glance at Table XXV. will show that, on the whole, 

 melting-point and formula grow together. It may not im- 

 probably prove to be the case that, when the whole subject of 

 melting-point has been successfully investigated, this simple 

 relation is the limiting condition of the real law. The data, 

 however, hitherto adduced are far from adequate to a discus- 

 sion of numerical relations among melting-points: for such an 

 object it would be a fruitless task to examine them further. 



Some negative results of this investigation are worthy of 

 attention. Thus u and /3 nitrophenol have the same additive 

 formula, and yet differ by 67 o- 063 in their melting-point. It 

 is clear then, as already well known, that melting-point may, 

 in cases of isomerism, be related to something else than for- 

 mula. The melting-point of naphthalin is actually lowered in 

 the first stage of nitration. Considerations such as these may 

 perhaps serve as suggestions for future work. 



An accurate method of determining melting-point places it 

 within our power to detect far more delicate shades of isomeric 

 differences than have hitherto been regarded as possible. 

 Thus strong presumptive evidence has been adduced (p. 14) 

 that dinitrotoluol, when prepared directly from toluol or from 

 liquid (meta-) nitrotoluol, or from solid (para-) nitrotoluol by 

 gentle nitration, melts at 69°"252; but that when paranitro- 

 toluol is energeticaly nitrated, the product melts at 69 c '610. 

 There are consequently two modifications of dinitrotoluol ob- 

 tainable very directly from toluol — the melting-point of these 

 substances differing by O- 358, a quantify far beyond the 

 range of error of the method. In like manner, it can be 

 shown that two parallel modifications of trinitrotoluol exist — 

 one of which melts at 78°-853, the other at 80°'532 * 



[For a complete account of the thermometers referred to in 

 this memoir the reader is referred to the Transactions of the 

 Eoyal Society of Edinburgh, 1881, p. 567; for the method of 

 determining melting-point, to the Proceedings of the Roval 

 Society, vol. xxxiii. p. 203.] 



* Compare Phil. Mag. 1875, 1. p. 17. 



