248 Mr. C. Tomlinson on the Action of Solid Nuclei [Jan. 21, 



very little water. If the flame of a spirit-lamp be applied to the bottom 

 of the tube, the crystals soon fuse and throw down a portion of the anhy- 

 drous salt, which is highly favourable to the production of soubresauts. 

 If the tube be suspended against an upright surface, with a mark opposite 

 the mouth, it will be easily seen that every burst of steam is accompanied 

 by a violent downward jerk. 



In order to mitigate or prevent .this bumping ebullition, it has long been 

 the practice to introduce into the retort or other vessel, a few angular 

 pieces of solid matter, metallic being the best — such as platinum-foil, silver, 

 copper, or platinum-wire or filings, fragments of cork or torn cartridge- 

 paper. Faraday names these substances " promoters of vaporization," 

 without explaining their action ; and he remarks that if any one of these 

 substances be suddenly introduced, " it is probable that the consequent 

 burst of vapour would be so instantaneous and strong as to do more harm 

 than the bumping itself" *. This is precisely the action of an unclean 

 solid introduced into a supersaturated gaseous solution, or in the case of a 

 liquid at or near the boiling-point, into a supersaturated vaporous solution. 



When sand, fragments of glass, or other non-metallic substances are 

 used for preventing bumping, they facilitate the escape of vapour only so 

 long as they are unclean ; but as siliceous bodies are readily cleansed by 

 the action of boiling water and other boiling liquids, they often aggravate 

 the evil. For example, two ounces of distilled water containing a little 

 sand from the sand-bath, were boiled in a six-ounce German flask over a 

 spirit-lamp. The boiling proceeded briskly without any kicking. The 

 lamp was removed and the flask left to cool. Next morning the lamp was 

 again put under the flask, when the water boiled with such violent kick- 

 ings as to endanger the safety of the vessel. The sand had become che- 

 mically clean during the first boiling. If sand, cleaned by means of 

 sulphuric acid and much rinsing, be added to water in the first instance, 

 the kickings set in at once. Similar results were obtained with fragments 

 of glass ; when chemically clean, they serve to enlarge the adhesion surfaces, 

 instead of the vapour-giving surfaces, and so increase the resistance to be 

 overcome. 



With respect to the action of metals, there is no advantage in making 

 them sharp-pointed, nor in having their surfaces rough ; only, in the latter 

 case, unclean vapour-giving substances are apt to lodge in the rough lines, 

 or between the teeth, and so far a file or other rough body may be of ad- 

 vantage. Metal filings are also liable to collect dust and specks of dirt, 

 which act as nuclei. The following experiment shows the action of clean, 

 as compared with unclean iron-filings. A flask cleaned by means of sul- 

 phuric acid contained four ounces of distilled water, which boiled at 215°. 

 Some iron-filings that had long been kept in spirits of wine were thrown in. 

 There was a good deal of kicking, and the temperature oscillated between 

 213^° and 213^°. Some unclean filings were thrown in, and the effect 

 * Chemical Manipulation, p. 200. 



