476 



Prof. Guthrie on Drops, 



[Recess, 



these two ingredients were present. We could scarcely approach to an 

 answer by any of the means hitherto employed. The specific gravities of 

 the two liquids are so close ('864, '863) that the density of the mixture 

 would give us no substantial aid. Though there is a considerable differ- 

 ence (80° C.) in their boiling-points, no one who is familiar with the diffi- 

 culties of fractional distillation would place any reliance upon a quantita- 

 tive separation based upon volatility. Their refractive indices are nearly 

 the same*. Their vapour-densities, 2*77, 4*76, though comparatively 

 different, are not absolutely very wide apart. They are active and passive 

 towards most of the same chemical reagents, and interfere with one another's 

 reactions. If we have recourse to chemical analysis (C^2 Hg, H^g), a very 

 small experimental error would point to a great difference in the proportion 

 of the two. 



To find how far the stalagmometer (Plate V. fig. 7) is applicable in this 

 case, it was filled with five liquids in succession : — 



1st, with benzol , . . =B. 



2nd, with two volumes benzol and one of turpentol =B2T. 



3rd, with one volume benzol and one of turpentol. . =BT. 



4th, with one volume benzol and two of turpentol —WT^, 



5th, with turpentol =T. 



The time-growth being brought in each case to 5", the number of drops 

 of water required to fill a given volume was counted, allowance behig made 

 for the meniscus. 



Table XVI. 



Air. 



T. 





BT. 



B^T. 



B. 



102 



51 



38 



34 



31 



14 



102 



51 



37 



33 



31 



14 



101 



50 



38 



33 



31 



14 





49 











101-7 



50-2 



37-7 



33^ 



31 



14 



Mean. . 



Hence a difference of 16*6 per cent, in one of the constituents corre- 

 sponds to an observed difference, under the most unfavourable conditions, 

 of three drops. In other words, the stalagmometer is sensitive to an alter- 

 ation of about 6 per cent. By increasing the capacity of the recipient, it is 

 clear that the drop-numbers, and therefore their differences, might be in- 

 creased at pleasure. Thus by counting the number of drops necessary to 

 fill a volume six times the size, we could tell to within one per cent, how 

 much turpentol and how much benzol were present. 



But it is perhaps in the cases of the still more proximate identity of iso- 

 meric bodies mentioned above that the stalagmometer may be used rather 



* The refractive index of turpentol is 1-476; that of benzol does not appear to have 

 been measured ; but that it is almost identical with that of turpentol is seen on mixing 

 the two. In those cases in which I propose chiefly to use the stalagmometer, namely 

 with isomeric liquids, the method of refraction is useless, because isomeric liquids 

 seem always to have the same refractive indices. 



