240 



BOILING OF LIQUIDS. 



BOLETUS, MEDICAL USES OP. 



250 



instead of employing platinum vessels it is sufficient to introduce a 

 piece of platinum into the liquid which is in a glass vessel. 



On ascending mountains, by the consequent diminution of atmo- 

 spheric pressure, and in proportion to it, water is found to boil at a 

 lower temperature. Thus, on the summit of Mont Blanc, which is 

 15,650 feet above the level of the sea, Saussure found water to boil at 

 185'8 of Fahrenheit. M. Wisse has recently determined the boiling 

 point on Mount Pichincha, at an altitude of 15,940 feet, to be 

 185'27 Fahr., whilst the barometer stood at 17'20S inches. 



The effect of diminished pressure hi lowering the boiling point may 

 be readily exhibited : remove some boiling water from the fire, and 

 ebullition sooii ceases, but it is renewed by placing it under the receiver 

 of an air-pump and quickly exhausting the air. Another and very 

 simple method of producing the same effect is, to boil some water in a 

 Florence flask, cork it while boiling, remove it immediately from the 

 fire, and immerse it almost entirely in cold water, when the ebullition 

 will recommence. This is occasioned by the sudden condensation of 

 the steam which occupied the upper part of the flask, and the conse- 

 quent formation of a vacuum ; the existence of which is proved by the 

 rush of air into the flask on removing the cork. 



According to the Kev. Mr. Wollaston ('Phil. Trans.,' 1817), an 

 elevation of 530 feet causes a diminution of 1 of Fahrenheit in the 

 temperature of boiling water ; but it will be observed that this deter- 

 mination, which is probably an accurate one, does not agree with the 

 stated height of Mont Blanc, or the temperature at which water boils 

 on its summit. 



The Rev. Mr. Wollaston contrived an apparatus for determining 

 elevations by the temperature of boiling water. It has been improved 

 and introduced under the name of the Barn- 

 metrical T/termnmetfr, made by Negretti and 

 Zambra, to meet the requirements of tra- 

 vellers under circumstances where the mer- 

 curial barometer cannot be conveniently 

 employed. The instrument is very portable, 

 and consists of a very delicate thermometer, 

 about 12 inches long, with a scale ranging from 

 180 to 212, each degree being subdivided so 

 as to show distinctly 0'l. There is also a 

 copper boiler c attached to a small tripod 

 stand ; from the top of the boiler proceeds a 

 three-draw telescopic tube, open at the top ; 

 this tube is sxtrrounded by a second, similarly 

 constructed, and screwed at the top of the 

 boiler ; the outer tube has two openings, one 

 at the top through which the thermometer is 

 inserted, passing down to within an inch of 

 the water in the boiler, and supported by 

 means of a cork or India-rubber washer ; the 

 second opening, forming an nutlet for the 

 steam, is shown at o. The object of the 

 double tube is to ensure a steady boiling-pi >int. 

 which cannot well be obtained in open air 

 experiments, where only a single tube is 

 employed. A is a metallic spirit-lamp, sur- 

 rounded with wire-gang!', to prevent the flame 

 from being extinguished when experimenting 

 in the open air. Each instrument is furnished 

 with a carefully computed set of tables, from 

 which may be obtained, by an easy calculation, 

 the elevation corresponding to any observed 

 boiling-point, between the temperatures of 

 180 ami 212. In using the apparatus, a suffi- 

 cient quantity of water is poured into the 

 boiler, through the small opening F, to fill it 

 about one third, when it is closed by means of 

 a screw ; the spirit-lamp is then applied, and 

 when the water boils, the steam rises, .-nr- 

 n Hindi) the bulb and tube, descends between 

 the two tubes, and issues from the opening at 

 !i. After a few seconds the mercury hi the 

 thermometer will rise and become stationary ; 

 the degree indicated by it must then be noted, when, by reference to 

 the tables, the elevation of the spot where the experiment has been 

 performed may be obtained. The temperature of the air should be 

 observed by ;i common thermometer at the -ame time. 



'sgor Robison states that fluids boil in vacuo at 140 lower than 

 under atmospheric pressure ; consequently, water so circumstanced will 

 Ixiil at 72. Dr. Thomson informs us that he has seen water boiling 





in Mr. Barry's apparatus for distilling oils in vacuo. 

 have now described the circumstances under which the boiling 

 point of water is lowered by diminishing the pressure; and we shall 

 proceed to show how, by increasing the pressure, the boiling point is 

 raised. 



When water is heated in vessels from which the vapour cannot 

 escape except by overcoming pressure, its boiling point becomes very 

 much raised. This experiment may be made in I'uj>in' ill'/fxtcr, which 

 i a strong iron or copper vessel, with a tight-fitting lid screwed down. 

 and provided with a safety- valve, loaded with any proper quantity of 



weights. In this way water may be heated to upwards of 400 ; 

 indeed, according to Muschenbroek, the temperature of water can be 

 raised so as to melt tin, which fuses at 442. A more convenient 

 apparatus for this purpose was invented by the late Dr. Marcet. In 

 this, the pressure is indicated by the height to which the steam raises 

 a column of mercury, and the temperature is shown by a thermometer. 

 It is represented in most treatises on Heat, but we may refer to 

 Professor Miller's ' Chemistry,' vol. i. p. 227. 



It should be observed that the temperature of the steam is always 

 equal to that of the water from which it is generated. Wh ju however 

 what is termed high-pressure steam is suffered to escape into the atmo- 

 sphere, its temperature is greatly reduced, not merely on account of ' 

 the cold air with which it comes into contact, but from the great 

 expansion which it undergoes, and the consequent conversion of sensible 

 into latent heat. In this case, it is so far from scalding like atmospheric 

 steam, that it may be received upon the hand without feeling un- 

 pleasantly hot. When water is boiled in vessels which are not furnished 

 with safety valves, or when from any accident they do not act or are 

 overloaded, the strongest boilers burst with a tremendous explosion. 



There are several circumstances which influence the boiling point of 

 water besides those already noticed, though not to so great a degree. 

 M. Gay Lussac found that water boiled exactly at 212 in a vessel 

 made of tin plate, while in a glass one it acquired 214 ; and he con- 

 cluded that the boiling point varies according to the nature of the 

 different vessels, and the state of their surfaces, in which the ebullition 

 takes place, and consequently depends on their conducting power and 

 the polish. It has been since ascertained that adhesion of the fluid to 

 the surface of the vessel is the chief circumstance concerned in raising 

 the boiling point. Thus, water may boil at 214 in a glass vessel, but 

 if a pinch of iron filings be allowed to fall in, the adhesion will be 

 diminished, and the temperature will sink to 212. If the inner surface 

 of the vessel be varnished with shell-lac, the temperature of 221 may 

 be attained before the water boils ; but the temperature of each gusli 

 of vapour is not more than 212. The presence of a small quantity of 

 oil in water has the effect of raising its boiling point 3 or 4. The 

 presence of air in liquids diminishes the adhesion, and greatly assists 

 the evolution of vapour. By long boiling, so as to get rid of the whole 

 of the air, Donny was able to raise its temperature in an open glass 

 vessel as high as 360 Fahr., when it suddenly exploded into vapour 

 with a loud report. 



Dr. Bostock also found (' Annals of Philosophy,' vol. xxv. p. 196) that 

 the boiling point of water is materially influenced by the presence of 

 extraneous bodies. A saturated solution of common salt was placed 

 over a lamp, and gradually heated up to 222, when it boiled strongly ; 

 a test tube, containing water deprived of air by boiling, was plunged 

 into the heated brine, and in a second or two it began to boil ; the 

 lamp was then withdrawn, and the brine soon ceased to boil, but the 

 ebullition continued in the water for some time longer ; it subsided at 

 about 218 or 217, but was constantly renewed by dropping in pieces 

 of cedar wood. The brine was again placed over the lamp, and a test 

 tube was plunged into it, containing a portion of water, together with 

 a thermometer. The water in the tube did not begin to boil until the 

 thermometer had risen to between 216 and 217 D , when ebullition first 

 commenced ; the fragments of wood were then dropped in, and as 

 usual very much increased the ebullition ; and it was found that the 

 water, kept at this temperature, had its ebullition promoted or sus- 

 pended, according to the presence or absence of the extraneous bodies. 

 Dr. Bostock concludes that in water the difference of boiling point 

 occasioned by the circumstances described amounts to 4 or 5, but in 

 ether occasionally to 50 or more. 



BOLE, ARMENIAN. [BOLE, in NAT. HIST. Div.] 



BOLETIC ACID. [FUMAKIO ACID.] 



BOLETUS, MEDICAL USES OF. Several different species, all 

 confounded under the name /?. igntariux, furnished the means of 

 stanching the flow of blood from wounds. They were supposed to do 

 this by an astringent property ; and, being erroneously referred to the 

 genus Agaricus, were termed ayaric, which word is often used as 

 synonymous with ti/ptir. Boletus possesses, however, no peculiar 

 power of arresting the flow of blood, but acts mechanically like a 

 sponge, and favours the formation of a clot. It is now almost entirely 

 disused by British surgeons, but in some cases it merits a preference 

 over other means of closing a bleeding vessel. When it is to be used, 

 it must be rubbed firmly between the hands, doubled, and applied 

 over the orifice whence the blood proceeds, and bound down by a com- 

 j press. It should not be removed till after twenty-four hours, and the 

 clot should be softened with cold, not warm water. Though the 

 ' German tinder seems to offer a convenient substitute for the prepared 

 agaric in ease of an emergency, it would be very improper to employ 

 it, as the nitrate of potass or saltpetre in which it is steeped would 

 irritate and inflame the edges of the wound. [AMADOU, in NAT. HIST. 

 Div.] The German tinder, however, forms a very excellent moxa. 

 The different kinds of Boleti used as styptics were formerly designated 

 Af/aricus chinti'f/firuin. 



It is less on account of their uses than of their peculiar habitudes, 

 that the Boleti merit our notice. In chemical composition, odour, and 

 habitudes, they resemble animals more than vegetables. When cut 

 into, some of them exhibit almost a muscular structure (Ji. Itfyatims, 

 or Fistulina hf^iatica), hence called by the French lawjuc de bceuf. The 



