6ro 



SCIENTIFIC NEWS. 



[Dec. 14, 1888. 



In the first part of the paper attention was again 

 called to the misleading results of the common ap- 

 plication of the theory of the perfect engine as a basis 

 for an index to the comparative efficiency of actual 

 engines, working under different conditions and with 

 different pressures. Examples of the reasoning based 

 upon this interpretation of the "Carnot " theorem, 

 or this application of the " Carnot " ratio, were given 

 as showing that it had been common to suppose that 

 the useful limit to steam pressure would be reached 

 at about 200 lbs. or below it. Reasons for expecting 

 very large increase in available work as due to the in- 

 crease of pressures up to at least 300 lbs. were given 

 with figures showing the extra work available per extra 

 unit of heat used. 



In dealing with the steam engine as a heat engine, 

 it was argued that considerations relative to the boiler 

 as the producer of the steam and as the receiver of the 

 hot water rejected by the engine are irrelevant ; and, 

 further, that no part of the history of the steam used 

 should be included other than that part which begins 

 with the admission of the steam into the cylinder and 

 ends with its emission therefrom. 



In estimating the quantity of steam required to per- 

 form a given quantity of work, the steam necessary to 

 fill the cylinder at the pressure shown by indicator 

 diagrams was not credited with providing any of the 

 heat necessary for the performance of the work, except- 

 ing only the units of heat represented by the difference 

 between the total heat of the steam at the initial and 

 at the terminal pressures. In a non-expansive engine 

 the whole of the heat equivalent of the work done would 

 have, according to the paper, to be supplied in addition 

 to that represented by the volume of steam required to 

 fill the cylinder. 



This heat was assumed to be supplied by initial 

 condensation, and an object of the paper was to show 

 that the performance of work alone demands sufficient 

 heat to account (with the exception of some very small 

 losses) for the whole of the observed initial cylinder 

 condensation. 



Proceeding on this thesis it was argued that cylinder 

 condensation apart from this is a very small quantity 

 in the best actual engines, and that it consists mainly 

 in loss of heat by evaporation during exhaust, and of 

 slight losses due to radiation and conduction, and to 

 clearance space not filled at initial pressure by com- 

 pression. 



From this it was concluded — 



1. That as small a portion ot the cylinder capacity as 

 possible should be subject to exhaust influences. 



2. That it follows from 1 that multiple stage expansion 

 is necessary with the high pressures that are advisable, 

 and that high speeds of rotation ought to be attended 

 with economy. 



BELFAST NATURAL HISTORY AND PHILO- 

 SOPHICAL SOCIETY. 

 At a meeting held on Tuesday, Dec 4th, a lecture en- 

 titled "Soap Bubbles" was delivered by Mr. John 

 Brown. 



Professor Letts (the President) occupied the chair. 

 The lecturer said — The record of men of science who 

 have devoted attention to the phenomena connected with 

 soap bubbles is a pretty large one, and includes such 

 men as the celebrated Robert Boyle (who, in 1663, 

 examined and tried to account for the colours of the 



bubble), Sir Isaac Newton, Hooke, Young, Leidenfrost, 

 Brewster, Draper, Sir Wm. Thomson, and many others, 

 as well as the great French physicist, Plateau, whose 

 researches have a sadly pathetic interest, since he was 

 in the midst of them attacked by a disease which deprived 

 him of sight, and it was only with " that inner eye which 

 no calamity could darken " that he continued to investi- 

 gate these beautiful phenomena as their colours and 

 forms were described to him by his two faithful friends 

 and assistants, Duprez and Donny. The soap bubble 

 is of interest, first, on account of the peculiarities of its 

 own structure, its forms, its durability, and its colour ; 

 and, secondly, as an experimental tool for the investiga- 

 tion of other physical phenomena. The round form of a 

 free bubble is due to the interaction of the air pressure 

 inside it and the surface tension or elasticity of the liquid 

 film. Both being equal all over the surface, it can be 

 shown how no other figure could satisfy the conditions. 

 The elasticity of the film is illustrated by hanging a 

 bubble to a fixed wire ring and hanging to the bubble a 

 second ring with a small weight attached. On allowing 

 the air to escape by breaking the film inside the fixed 

 ring the shrinking up of the film lifts the weight up to 

 the upper ring. The colours of the bubble were for a 

 long time a great puzzle to the early investigators, but 

 since the discovery of the undulatory theory of light a 

 satisfactory explanation of them has been found. They 

 belong to a class of phenomena known as " colours of 

 thin plates," and are caused by the " interference " of the 

 light waves, reflected from the outer surface of the film 

 with those reflected from the inner surface through the 

 thickness of the film. The effect is to destroy some one 

 colour in the light, leaving its complementary colour un- 

 balanced, and therefore visibly tinting the otherwise 

 white light. The matter is very clearly explained in 

 Tyndall's charming book, " Lectures on Light." The 

 colours are very well seen on a flat soap film attached to 

 a ring of wire, or in a bubble partly filled by smoke, as 

 was observed by Leidenfrost in the last century He 

 compares the resulting effect to a brilliant star, and then 

 he moralises quaintly, " But all this glory vanishes the 

 moment the bubble bursts ; the fetid smoke which 

 escapes shows with what filth it was filled, and so offers 

 us a striking picture of the gilded miseries of hu- 

 manity." The well-known fact that two bubbles may 

 be pressed together without coalescing is well illustrated 

 by blowing one bubble inside another one hanging on a 

 wire ring. The outer one must have either a drop of 

 liquid or a wire ring hung to its under part, so as the 

 thick under part of the inner one does not touch it ; 

 otherwise they will coalesce : or if the inner one be 

 filled with coal gas it rises to the top of the outer one, 

 which answers the same purpose. When the ring on 

 which the outer one is fixed is very light and small the 

 whole combination rises in the air, the inner bubble 

 representing the bag of gas, the outer one the net, and 

 the ring the car of the miniature balloon. As an ex- 

 perimental tool the bubble may be used to illustrate the 

 diffusion of gases, a phenomenon examined and investi- 

 gated by Graham, who, like other great pioneers of 

 science, attacked the most abstruse problems with the 

 simplest possible apparatus. It has been stated that his 

 entire laboratory for his diffuson experiments was con- 

 tained on an old tea tray, and consisted of a few glass 

 tubes, phials, and tobacco-pipe stems, and some plaster 

 of Paris. After all, as in the case of the astronomical 

 telescope, it is the man at the small end that is of most 



