of a Liquid Mass without Weight. 447 



general characters of the first are the formation of little or no 

 froth when shaken, the incapability of being blown into bubbles, 

 the absence of colours on the hemispheric bubbles, or a tardy 

 and only incipient coloration, showing only the red and green of 

 the last orders. Among the numerous liquids which belong to 

 this category, I may mention water, glycerine, sulphuric and 

 nitric acids, ammonia, saturated solutions of tartaric acid, nitrate 

 of potassium, carbonate of sodium, and chloride of calcium. 



The liquids of the second category are distinguished from the 

 preceding by the prompt and decided coloration of their films, 

 showing tints of all the orders. These liquids are the fat 

 oils, lactic acid, glacial acetic acid, oil of turpentine, alcohol, 

 benzine, Dutch liquid, chloroform, sulphuric ether, sulphide of 

 carbon*, and no doubt many more. 



The liquids which belong to the third category are covered 

 over, when shaken, with an abundant and very persistent froth ; 

 they can be easily blown into bubbles at the end of a pipe; the 

 hemispherical bubbles which they form last much longer than 

 those formed by the liquids of the two preceding categories, usu- 

 ally for several hours, and sometimes even for several days. They 

 have generally at first a well-marked colourless phase,, the dura- 

 tion of which differs much in different liquids ; they then become 

 gradually coloured, but in a way which varies somewhat with 

 the nature of the liquid. 



This category is not numerous : if we take away some sub- 

 stances which are only liquid when hot, such as glass, it is re- 

 duced essentially, I think, to the solutions of different kinds of 

 soap, of saponine, and albumen, to which may be added solution 

 of sesquiacetate of iron. 



In order not to make this abstract too long, I omit a series of 

 curious facts that have been met with in the course of the expe- 

 riments, and an account of which will be found in the memoir. 

 I pass on to the deductions which have an immediate bearing 

 upon the question I am discussing. 



We have seen that films of the second category assume, im- 

 mediately on their formation or very soon afterwards, bright 

 colours belonging to all the orders; whence we must conclude 

 that they get thinner with extreme rapidity. 



We have seen also that there is never an immediate or nearly 

 immediate coloration in the films of the first category : the very 

 great majority remain colourless till they break ; in the very 

 rare cases in which such films do become coloured, this does 



* At ordinary temperatures, the hemispherical bubbles of sulphide of 

 carbon, which last only a fraction of a second, do not exhibit colours; but 

 at a few degrees below zero a bright coloration may be observed on some 

 of them. 



