576 



NA TURE 



^Fcb. 17, 1 88 1 



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When, injitead of breaihing iuto a dry flask, the common air 

 of the laboratory was urged through it, the sounds became imme- 

 diately intensified. I was by no means prepared for the extra- 

 ordinary delicacy of this new method of testing the athermancy 

 and diathermancy of gases and vapours, and it cannot be otherwise 

 than sitisfactory to me to find that particular vapour, whose 

 alleged deportment towards radiant heat has been most strenuously 

 denied, alarming thus audibly its true character. 



After what has been stated regarding aqueous vapour we are 

 prepared for the fact that an exceedingly small percentage of any 

 highly athermanous gas diffused in air suffices to exalt the 

 sounds. An accidental observation will illustrate this point. A 

 flask was filled with coal gas and held bottom upwards in the 

 intermittent beam. The sounds produced were of a force cor- 

 responding to the known absorptive energy of coal-gas. The 

 flask was then placed upright, ^\ ith its mouth open upon a table, 

 and permitted to remain there for nearly an hour. On being 

 restored to the beam, the sounds produced were far louder than 

 those which could be obtained from common air.'^ 



Transferring a small flask or a test-tut e from a cold place to 

 the intermittent beam it is sometimes found to be practically 

 silent for a moment, after which the sounds become distinctly 

 audible. This I take to be due to the vaporisation by the 

 calcrific beam of the thin film of moisture adherent to the glass. 



My previous experiments having satisfied me of the generality 

 of the rule that volatile liquids and their vapours absorb the 

 same rays, I thought it probable that the introduction of a thin 

 layer of its liquid, even in the case of a most energetic vapour, 

 would detach the efl'ective rays, and thus quench the jounds. 

 The experiment \\ as made and the conclusion verified. A layer 

 of water, formic ether, sulphuric ether, or acetic ether one-eighth 

 of an inch in thickness rendered the transmitted beam powerless 

 to produce any musical sound. These liquids being transparent 

 to light, the efficient rays which they intercepted must have been 

 those of obscure heat. 



A layer of bisulphide of carbon about ten times the thickness 

 of the transparent layers just referred to, and rendered <ipa'|ue 

 to light by dissolved iodine, was interposed in the path of the inter- 

 mittent beam. It produced hardly any diminution of the sounds 

 of the more active vapours — a further proof that it is the invisible 

 heat rays, to which the solution of iodine is so eminently trans- 

 parent, that are here effectual. 



Converting one of the small flasks used in the foregoing 

 experiments into a thermometer bulb, and filling it with various 

 gases in succession, it was found that with those gases which 

 yielded a feeble sound, the displacement of a thermometric 

 column associated with the bulb was slow and feeble, while with 

 those gases which yielded loud sounds the displacement was 

 prompt and forcible. 



Further Experiments. — Since the handing in of the foregoing 

 note, on January 3, the experiments have been pushed forward ; 

 augmented acquaintance with the subject serving only to confirm 

 my estimate of its interest and importance. 



All the results described in my first note have been obtained 

 in a very energetic form Hith a battery of sixty Grove's cells. 



On January 4 I chose for my source of rays a powerful lime- 

 light, which, when sufficient care is taken to prevent the pitting 

 of the cylinder, works with admirable steadiness and v ithout 

 any noise. I also changed my mirror for one of shorter focus, 

 which permitted a nearer approach to the source of rays. Tested 

 with this new reflector the stronger vapours rose remarkably in 

 sounding power. 



Improved manipulation was, I considered, sure to extract 

 sounds from rays of much more moderate intensity than those of 

 the lime-Iight. For this light, therefore, a common candle tlanie 

 was substituted. Received and thrown back by the mirror, the 

 radiant heat of the candle produced audible tones in all the 

 stronger vapours. 



Abandoning the mirror and bringing the candle close to the 

 rotating disk, its direct rays produced audible sounds. 



A red-hot coal, taken from the fire and held close to the 

 rotating disk, produced forcible sounds in a flask at the other 

 side. 



A red-hot poker, placed in. the 1 osition previously occupied 

 by the coal, produced strong sounds. Maintaining the flask in 

 position behind the rotating disk, amusing alternations of sound 

 and silence accompanied the alternate introduction and removal 

 of the poker. 



I The method here described i?, I doubt not, applicable to the detection 

 of extremely small quantities of fire-damp ia mines. 



The temperature of the iron w as then lowered till its heat just 

 ceased to be visible. The intermittent invisible rays produced 

 audible sounds. 



The temperature was gradually lowered, being accompanied 

 by a gradual and continuous diminution of the sound. When it 

 ceased to be audible the temperature of the poker was found tc 

 be below that of boiling water. 



As might be expected from the foregoing experinents an 

 incandescent platinum spiral, with or without the mirror, pro- 

 duced musical sounds. When the battery power was reduced 

 from ten cells to three the sounds, though enfeebled, were still 

 distinct. 



My neglect of aqueous vapour had led me for a time astray in 

 1859, but before publishing my results I had discovered my 

 error. On the present occasion this omnipresent substance had 

 also to be reckoned with. Fourteen flasks of various sizes, with 

 their bottoms covered with a little sulphuric acid, were closed 

 with ordinary corks and permitted to remain in the laboratory 

 from December 23 to January 4. Tested on the latter day with 

 the intermittent beam, half of them emitted feeble sounds, but 

 half were silent. The sounds were undoubtedly due, not to dry 

 air, but to traces of aqueous vapour. 



An ordinary bottle containing sulphuric acid for laboratory 

 purposes, being connected with ihe ear and placed in the inter- 

 mittent beam, emitted a faint, but distinct, musical sound. This 

 bottle had been opened two or three times during the day, its 

 dryness being thus vitiated by the mixture of a small quantity of 

 common air. A second similar bottle, in which sulphuric acid 

 had stood undisturbed for some days, was placed in the beam : 

 the dry air above the liquid proved absolutely silent. 



On the evening of January 7 Prof. Dewar handed me four 

 flasks treated in the following manner : — Into one was poured a 

 small quantity of strong sulphuric acid ; into another a small 

 quantity of Nordhausen sulphuric acid ; in a third were placed 

 some fragments of fused chloride of calcium ; while the fourth 

 contained a small quantity of phosphoric anhydride. They were 

 closed with well-fitting india-rubber stoppers, and permitted to 

 remain undisturbed throughout the night. Tested after twelve 

 hours, each of them emitted a feeble sound, the flask last-men- 

 tioned being the strongest. Tested again six hours later, the 

 sound had disappeared from three of the flasks, that containing 

 the phos; horic anhydride alone remaining musical. 



Breathing into a flask partially filled with sulphuric acid in- 

 stantly restores the sounding power, which continues for a 

 consideratde time. The wetting of the interior surface of the 

 flask with Ihe sulphuric acid always enfeebles, and sometimes 

 destroys, the sound. 



A bulb less than a cubic inch in volume, and containing a 

 little water low ered to the temperature of melting ice, produces 

 very distinct sounds. Warming the water in the flame of a 

 spirit-lamp, the sound becomes greatly augmented in strength. 

 At the boiling temperature the sound emitted by this small bulb ' 

 is of extraordinary intensity. 



Tbese results are in accord with those obtained by me nearly 

 nineteen years ago, both in reference to air and to aqueous 

 vapour. They are in utter disaccord with those obtained by 

 other experimenters, who have ascribed a high absorption to air 

 and none to aqueous vapour. 



The action of aqueous vapour being thus revealed, the neces- 

 sity of thoroughly drying the flasks when testing other sub- 

 stances becomes obvious. The following plan has been found 

 effective : — Each flask is first heated in the flame of a spirit- 

 lamp till every visible trace of internal moisture has r isappeared, 

 and it is afterwards raised to a temperature of about 400° C. 

 While the glass is still hot a glass tube is introduced into it, and 

 air freed from carbonic acid by caustic potosh, and from aqueous 

 vapour by sulphuric acid, is urged through the flask until it is 

 cool. Connected with the ear-tube, and exposed immediately 

 to the intermittent beam, the attention of the ear, if I may use 

 the term, is converged upon the flask. When the experiment is 

 carefully made, dry air proves as incompetent to produce sound 

 as to absorl) radiant heat. 



In 1S68 I determined the absorptions of a great number of 

 liquids whose vapours I did not examine. My experiments 

 having amply proved the parallelism of liquid and vaporous 

 absorption, I held undoubtingly twelve years ago that the vapour 

 of cyanide of ethyl and of acetic acid would prove powerfully 

 absorbent. This ccnclusion is now easily tested. A small 



■ In such bulbs even bisulphide of carbon vapour may be"so nursed as to 

 produce sounds of considerable strength. 



