626 



NATURE 



[October 26, 1893 



to pass a continuous current through the film from its first 

 formation. The measuring current was small and inter- 

 mittent. 



The conclusion to be drawn from this table was confirmed 

 by experiments in which transient currents from a Ruhmkorff 

 induction coil were employed ; they leave no doubt that the 

 passage through a film of such electrical currents as we have 

 used has no appreciable influence on the phenomenon under 

 discussion. 



To determine the possible influence of carbonic acid, or of 

 oxygen, absorbed from the surrounding space comparative 

 experiments were made. The apparatus not being air-tight, 

 the plan was adopted of allov/ing a stream of air carefully freed 

 from CO2, or of pure oxygen, as the case might be, to flow 

 through the film-box. The results obtained under these con- 

 ditions, though in some respecis not quite satisfactory, justify 

 the assertion that neither the total (or almost total) absence of 

 COo nor a large increase in the quantity of oxygen in the 

 neighbourhood of the film produces any appreciable change in 

 the specific resistance. 



It thus appears that a number of possible causes, to 

 which the increase in the conductivity of a thin film might 

 be due, prove on examination to have little or no influence. 

 Although a satisfactory explanation is not at present possible, 

 it will probably be found to depend upon the connection which 

 subsists between the chemical constitution of a film and the 

 surface forces which are brought into play, or are modified by its 

 tenuity. Prof. J. J. Thomson (" Applications of Dynamics to 

 Physics and Chemistry," p. 234) has drawn attention to this con- 

 nection, and has shown that under certain conditions the 

 chemical action in a thin film throughout which the forces pro- 

 during capillary phenomena are active, may be totally different 

 from the chemical action in the same substance in bulk. The 

 experiment of Liebreich {Berlin, Silzungsherichtc, 1886) is often 

 cited as illustrating this point. When solutions of chloral- 

 hydrate and sodium carbonate are mixed in suitable proportions 

 in a glass tube, chloroform is slowly precipitated as an opaque 

 cloud. Close to the surface, however, and from I to 3 mm. 

 below the surface, there is a space perfectly clear and free from 

 chloroform. It is supposed that in this space either the chemical 

 action does not go on, or that if it does chloroform is not de- 

 posited. The explanation is not very satisfactory, and in any 

 case the "dead space" is too large to justify us in referring it 

 solely to the action of surface forces. Again, there is no doubt 

 that the surface of a film becomes altered by the action upon it 

 of the .surrounding medium, so that the outer layers have diflierent 

 properties from the rest of the liquid. Lord Rayleigh has shown 

 that the surface tension of a soap solution when the surface is 

 new is nearly the same as that of pure water, but diminishes 

 rapidly by exposure to the air. Reinold and Riicker have proved 

 that the surface tension of a cylindrical film is increased by 

 giving the film a new surface (letting fresh liquid flow over it) 

 and that from ten to fifteen minutes elapse before the old value 

 is regained. Other properties of the surface-layer besides its 

 tension may be modified where it is very thin, and the electrical 

 conductivity may be very different from that of the interior 

 liquid. Although the peculiarities of the surface-layer certainly 

 are in some way connected with the main facts here considered, 

 we have shown that they cannot .nil of them be explained by the 

 simple theory of the formation of a pellicle of different con- 

 ductivity from the rest of the film. 



It is difficult to assign a reason why the addition of salt to the 

 liquid should produce so great a change in the results. In part, 

 the better conducting sail probably masks effects which, when 

 soap alone is used, become predominant ; but it is likely that, 

 in part at all events, it actually prevents the changes to which 

 the change in conductivity is due. 



The optical method of investigation illustrates the controlling 

 influence of the metallic salt when present in the solution. As 

 we saw above, a small variation in the proportion of dissolved 

 soap has a large effect upon the thickness of the black when no 

 salt is present ; but the quantity of soap may be doubled with- 

 out influencing the thickness, provided the solution contain 

 3 per cent, of salt. A. W. Rei.nold. 



NO. 1252, VOL. 48] 



SPONTANEOUS COMBUSTIONS 



"U17HEN an inflammable substance ignites or becomes incan- 

 descent without the application of fire or other apparent 

 cause, it has been customary to speak of it as spontaneous com- 

 bustion, a term which 1 think I shall be able to show you 

 presently does not correctly express the actions which lead to 

 this apparently mysteiious result. 



Early in the eighteenth century a woman was found burnt to 

 death under circumstances which gave no clue as to the cause of 

 the accident, and in order to satisfactorily explain her death, the 

 theory of spontaneous combustion was devised by the experts 

 of the day, and was generally accepted at a time when little or 

 nothing was known of what takes place during the process which 

 we know as combustion ; but as the years rolled on, men's views 

 upon this important subject became wider and more exact, until, 

 in the latter part of the last century, the great French philoso- 

 pher Lavoisier, partly by his own experiments, and partly by the 

 teachings of the work done by others, gave us a true knowledge 

 of combustion and the changes which take place when a body is 

 burnt, whilst the commencement of this century marked still 

 further the advance of our knowledge in this direction, and also 

 as to the conditions necessary for continuing the combustion or 

 burning of any inflammable substance. 



We now know that from the nature of combustion it is im- 

 possible for the human body to undergo spontaneous ignitioE or 

 combustion in the way in which the novelists and scientific ex- 

 perts of the last century believed possible, but there are few 

 amongst us who have not heard of, and even come across, cases 

 in which large masses of coal, small quantities of oily rags, or 

 waste, and hayricks which have been made from grass stacked 

 before it was thoroughly dried, have ignited without any apparent 

 cause, and have kept alive in our minds and on our tongues the 

 term " spontaneous combustion " ; and you must pardon me if I 

 commence my lecture this evening by reviewing the teachings of 

 Lavoisier's classical work, and then apply the conclusions we 

 arrive at to those cases of spontaneous combustion which we meet 

 with in our daily work. 



The theory of combustion which was generally accepted during 

 the last century, was that every combustible body contained 

 within itself the products of combustion combined with a 

 " something " called phlogiston, and when the substance was 

 burnt, this phlogiston escaped, giving the flame or incan- 

 descence of combustion, whilst the products were set f-ee. 

 This theory could not, however, for long stand the test of e.\act 

 experiment, and as soon as Black introduced the balance into 

 scientific research, it was found that when any substance under- 

 went combustion, the products weighed more than the body 

 before it had been burnt, the reverse of what one would have 

 expected had the phlogistic theory been correct. 



During the last century lived Joseph Priestley, one of the 

 most remarkable men this country has ever claimed as her own — a 

 man so varied in his attainments, and so energetic in his life and 

 labour, that he published over one hundred different works deal- 

 ing with every conceivable subject, from theology to science ; 

 but it was in the latter field that he especially shone, and the 

 greatest achievement of his life was the discovery of the gas 

 which we now call oxygen, a discovery which he communicated 

 to his friend Lavoisier. 



Lavoisier at once saw the importance of the discovery which 

 Priestley had made, and then conceived and carried out an ex- 

 periment which has become historical as proving for the first 

 time beyond doubt the fact that the air was not a simple ele- 

 mentary substance, but contained two perfectly distinct gases — 

 oxygen and nitrogen. 



Lavoisier placed in a long-necked retort about four ounces of 

 mercury, and so arranged the apparatus that the air above the 

 mercury in the retort should freely communicate with the air in 

 a measured receiver, all contact with the outer air being pre- 

 vented by standing the receiver in a vessel of mercury. He 

 now heated the four ounces of mercury in the retort nearly to 

 its boiling-point, and kept it at this temperature for twelve days 

 and twelve nights. At first no change took place, some of the 

 mercury merely distilling into the upper part of the appa- 

 ratus and falling back again ; but presently some little red 

 specks began to appear on the surface of the metal, and in- 

 creased in amount for several days, but at length ceased to 

 form : and after continuing the heating for a day or two longer, 



A lecture to working men, delivered by Prof. Vivian D. Lewes at 

 Nottingham, in connecticn with the British Association 



