418 THE FIGURES OF EQUILIBRimi OF A LIQUID MASS 



liquid ; tlie limpid portion is now to be drawn off by means of a siphon which is 

 primed with a lateral tube, and the preparation is terminated. It is proper to 

 add, however, that, when the short branch of the siphon is introduced into the 

 liquid, a portion of the deposit is brought away and forms around the exterior 

 surface of the tube a sort of reversed cone; it is necessary, therefore, before 

 priming the siphon, to rid it of this envelope. For this, the whole should be 

 first left at rest for a quarter of an hour, and then the immersed branch of the 

 siphon should be slightly shaken right and left ; the deposited cone will thus 

 become detached in small clots which gradually reascend and unite with the 

 upper layer. 2d process. — With the same care as in the former process, let one 

 volume of glycerine be mixed with three volumes of the solution of soap ; twenty- 

 four hours jifterwards filter, covering the funnel and renewing the filter when 

 the drops succeed each other only at long intervals ; finally, add to the filtered 

 liquor the quantity of glycerine necessary to establish the proportion of two vol- 

 umes of glycerine to three of the solution of soap, and the preparation is here 

 likewise terminated. This second process has the advantage of being somewhat 

 more expeditious than the first ; but if the filtering paper be not of excellent 

 quality, a sensible portion of the precipitate passes with the liquid, and the re- 

 sult is n(jt so satisfactory. 



The liquid thus prepared, and which I shall call glyceric liquid, yields films 

 of very great persistence : for example, a bubble having a diameter of one deci- 

 metre, deposited, in the open air of an apartment, on a ring of iron- wire of four 

 centimetres diameter, previously moistened with the same liquid, as in the ex- 

 periments which I shall presently describe, is capable, when in complete repose, 

 of lasting fully three hours. If the season be cold the second process is the 

 only one that can be employed, and days for operating must be chosen in which 

 the external temperature is several degrees above zero ; it is, moreover, necessary 

 to realize artificially, at least in an approximate degi'ee, the conditions of summer 

 with regard to the liquids. To tliis end, the apartment being heated to 66° or 

 68°, we must first, for an hour or two, keep the flasks which contain sepa- 

 rately the glycerine and the solution of soap in Avater maintained at the above 

 temperature; then the mixture is to be made; after which the flask containing it 

 should be placed in water at 66° or GS°; and we provide, whether by fire in the 

 apartment during the night or by enveloping the exterior vessel with thick folds 

 of woollen cloth, that the temperatm-e of the mixture shall descend but little 

 below 19^' for twenty-four hours. Filtration follows, the apartment being still 

 kept warm, and the preparation is finished as above stated ; the glycerine which 

 is added should itself have been kept, for about an hour, at a temperature of 66° 

 or 68". 



The glyceric liquid may be preserved for nearly a year ; it then, in a day or 

 two, becomes decomposed and completely loses its properties. This decompo- 

 sition has seemed to me to be unattended by any gaseous disengagement ; yet, 

 as the liquid is of an organic nature, it would seem not improbable that it 

 should sometimes be otherwise ; and it will be prudent, in order to avoid a pos- 

 sible explosion of the flask, to close the latter not too tightly with cork. Just 

 as the films formed with soap last much longer in a closed vessel than in the 

 free air, the duration of those of the glyceric liquid becomes much more con- 

 siderable when these films are enclosed in like manner, especially if certain pre- 

 cautions are used ; of this we shall see some examples at the end of the present 

 series, and I shall recur to this subject in a subsequent series. 



§ 14. For the realization of figures of revolution the following instruments 

 are required : 1st, a system of rings of non wire, seven centimetres in diameter, 



