Some Physical Properties of Nitric Acid Solutions. 89 



The cause of greasiness in burettes, a perpetual source of trouble in 

 accurate work, has been discussed by various writers, and attributed by 

 them to oils or fats in the distilled water derived from the lubricating 

 materials of the still (Ostwald), or to the vaseline, &c., used for lubri- 

 cating the stop-cock. Though probably both causes may be at work, 

 yet we consider that this greasiness is produced also by slime-forming 

 bacteria or hypomycetes, especially Dematium, the spores of which 

 settle and develop on the sides of the vessel ; our experience has shown 

 that if the air sucked in by the outflowing solution be filtered through 

 cotton-wool plugs, and if the burette be filled invariably from below, 

 and not, as is customary, from above, its state of cleanliness may be 

 greatly prolonged. 



The first table of specific gravities of nitric acid in terms of per- 

 centages was constructed by Kirwan ;* subsequent tables were 

 published by Ure,f Thenard and Kolb.J The numbers given by the 

 last are still reproduced more or less fully in text-books of chemistry 

 and physics, though it is now generally admitted that his results for the 

 more concentrated acids are too low. 



But as a considerable advance over previous writers, Kolb pointed 

 out, firstly, that the presence of the lower oxides of nitrogen alters 

 appreciably the value of the density ; secondly, that acid of approxi- 

 mately 100 per cent, concentration has a high coefficient of expansion; 

 and thirdly, that the maximum contraction on admixture with water 

 occurs at the percentage composition required for the hydrate 

 HN0 3 .3H 2 0. Upon the last result Kolb expressed himself as follows : 

 " Ce fait d'un maximum de contraction correspondant exactement a un 

 melange en proportions definies n'est pas un effect du hasard." The 

 most recent work upon the densities is that published by Lunge and 

 Key, which is now more generally quoted in technical manuals. 



The analytical methods of these last writers are similar in outline to 

 those herein adopted, but their final process of purification consisted in 

 distilling the concentrated acid with sulphuric acid in vacua, a method 

 which might cause slight contamination, as the vapour pressure of the 

 latter acid under these conditions could not be an insignificant 

 quantity. 



The densities were determined for 15/4, but it is not evident as to 

 those actually made, and those interpolated in order to reduce the 

 values to the commercial scale of Twaddell ; further, the results are 

 only taken to 0*001, which is far within the limits of accuracy 

 attainable. 



These writers, in conjunction with Marchlewski, have given a table 



* 'Irish Acad. Trans.,' vol. 4, p. 3 (1790). 



t ' Quart. Jourii. Sci.,' rol. 4, p. 291 (1818). 



t 'Ann. Chilli. Phjs.,' [4], vol. 10, p. 140. 



' Zeits. f. Angew. Chemie,' 1891, p. 167, and 1892, p. 10. 



