8 4 



NA TURE 



[A/ay 24, 1883 



on mixing with air and coming in contact with a flame, or 

 an incandescent body, the hydrogen sulphide would be 

 oxidised, and resolved in sulphur and water (with the 

 production of small quantities of sulphur dioxide) ; the 

 sulphur, minutely divided, would remain long suspended 

 in air, and cause the condensation to cloudy consistency 

 of the aqueous vapour. Piria illustrated his explanation 

 by a simple experiment : if in a vessel containing a mixture 

 of sulphuretted hydrogen and air a lighted taper is intro- 

 duced, a dense mist is rapidly formed ; a similar mist is 

 produced when glowing charcoal, or highly heated lava, 

 or pumice, or glass, or red-hot iron is introduced in the 

 gaseous mixture. When there is a large proportion of 

 ITS, the oxidation is very ripid, and the mixture explodes 

 and burns. 



Piria's explanation cannot be applied to the Bocca 

 delta Solfatara, where the presence of H 2 S cannot be 

 detected either by the sense of smell, or by the lead- 

 acetate test-papers. In the " Memorie Geologiche sulla 

 Campania {Rendiconti della Reale Accademia delle Scienze 

 lit Napoli, 1849, p. 137) Prof. A. Scacchi, after having 

 opposed Piria's opinion, gives the following explanation : 

 *' I believe the increase of the vapoury cloud due to the 

 carbonic acid produced in the combustion of the tinder, 

 its affinity for water causing the precipitation of the in- 

 visible vapour, and thus producing a mist." According 

 to Prof. Scacchi, in the presence of large quantities of 

 aqueous vapour, and at the temperature of the fumarole, 

 carbonic dioxide would act as hydrochloric acid gas 

 which fumes in ordinary air. 



Since 1849 no one (as far as I have gathered) has 

 suggested any new opinion or tried some experiment to 

 explain the phenomenon in question. I thought it would 

 i>e interesting to test experimentally at the Solfatara the 

 opinion of Prof. Scacchi. I was inclined to believe that, 

 if at the ordinary temperature carbonic dioxide does not 

 condense aqueous vapour from the air, there was very 

 little probability that the condensation would be caused 

 at temperatures as high as those of the vapours issuing 

 from the Bocca of the Solfatara (about CjC? Centigrade 

 externally) ; the action of flames or smouldering bodies 

 in augmenting the vapoury cloud appeared to me as 

 chiefly due to the condensation around the minute par- 

 ticles of soot or dust produced during the combustion. 



The following experiments were done during a clear 

 day, when abundant vapours were issuing from the large 

 fumarole : — 



1. A Wolff bottle (1 litre capacity), from which a con- 

 stant current of carbon dioxide was obtained (by pouring 

 dilute hydrochloric acid on marble fragments), was placed 

 on the ground inside the fumarole. The cloud of vapour 

 augmented. 



2. By means of a caoutchouc tube the C0 2 from the 

 generator was conducted near the hottest invisible 

 vapour. This vapour became interspersed with cloudlets 

 of condensed vapour, and the cloudy pillar outside the 

 Bocca greatly augmented. 



3. A large bottle (of about 15 litres capacity) filled with 

 carbon dioxide was brought inside the cavity, and the 

 C0 2 poured out. The effect was most striking outside 

 by the voluminous, but not immediate, outbursts of cloudy 

 vapour. 



4 With bellows of the kind used for sulphuring vines, 

 I blew sulphur dust inside the cavity. This caused the 

 production of great volumes of visible vapour. The same 

 effects were produced every time that minutely divided 

 bodies (wheaten flour, oxide of magnesia, chalky dust, 

 Sec.) were blown, or thrown, inside the cavity or near the 

 invisible vapour. 



5. The effect was very striking when the action of the 

 carbon dioxide (from the Wolff bottle) was combined with 

 the action of the sulphur dust. 



6. A small alcohol flame augmented the cloudiness of 

 the vapour. 



7. The smoky flame of burning naphthalene acted 

 much more powerfully than the alcohol flame. 



From these experiments, which (with the exception of 

 3 and 6) were often repeated, the following conclusions 

 may be drawn : — 



1. Carbon dioxide helps to condense watery vapour. 



2. Minute bodies suspended in air area powerful cause 

 (the principal cause, as Coulier and Aitken have shown) 

 in the condensation of aqueous vapour. 



3. The action of flames, or of incandescent bodies, in 

 augmenting so remarkably the volumes of visible vapour 

 rising from the fumaroles ot the Solfatara must be as- 

 cribed both to the carbon dioxide and to the minute car- 

 bonaceous particles set free during the combustion. 



( if these conclusions the first requires to be confirmed 

 by careful laboratory experiments. ITALO GlGLIOLI 

 Laboratory of Agricultural Chemistry, 

 R. Agric. College, Portici 



ST A TE OF THE A TMOSPHERE WHICH PRO- 

 DUCES THE FORMS OF MIRAGE OBSERVED 

 BY VINCE AND BY SCO RES BY 



IN 1 88 1, when I wrote the article Light tor the Encyc. 

 Britt, I had not been able to meet with any detailed 

 calculations as to the probable state of the atmosphere 

 when multiple images are seen of objects situated near 

 the horizon I had consulted many papers containing 

 what are called "general" explanations of the pheno- 

 mena, but had found no proof that the requisite conditions 

 could exist in nature : — except perhaps in the case of the 

 ordinary mirage of the desert, where it is obvious that 

 very considerable temperature differences may occur in 

 the air within a few feet of the ground. But this form of 

 mirage i essentially unsteady, for it involves an unstable 

 state of equdibrium of the air. In many of Scoresby's 

 observations, especially that of the solitary inverted 

 image of his father's ship (then thirty miles distant, and 

 of course far below the horizon), the details of the image 

 could be clearly seen with a telescope, showing that the 

 air must have been in equilibrium. The problem seemed 

 to be one well fitted for treatment as a simple example of 

 the application of Hamilti n's General Method in Optics, 

 and as such I discusse 1 it. The details of my investiga- 

 tion were communicated in the end of that year to the 

 Royal Society of Edinburgh, and will, I hope, soon be 

 published. The paper itself is too technical for the general 

 reader, so that I shall here attempt to give a sketch of its 

 contents in a more popular form. But a curious little 

 historical statement must be premised. 



It was not until my calculations were finished that I 

 found a chance reference to a great paper by Wollaston 

 {Phil. Trans. 1800). I had till then known only of Wol- 

 laston's well-known experiment with layers of different 

 liquids in a small vessel. But these, I saw, could not 

 reproduce the proper mirage phenomena, as the rays 

 necessarily enter and emerge from the transition strata 

 by their ends and not by their lower sides. This experi- 

 ment is by no means one of the best things in Wollaston's 

 paper, so far at least as the immediate object of the paper 

 is concerned. That so much has been written on the 

 subject of mirage during the present century, with only a 

 casual reference or two to this paper, is most surprising. 

 It may perhaps be accounted for by the fact that Wollaston 

 does not appear to have had sufficient confidence in his 

 own results to refrain from attempting, towards the end 

 of his paper, a totally different (and untenable) hypo- 

 thesis, based on the effects of aqueous vapour. Be the 

 cause what it may, there can be no doubt that the follow- 

 ing words of Gilbert were amply justified when they were 

 written, early in the present century : — " In der That ist 

 Wollaston der Erste und Einzige, der die Spieglung 

 aufwarts mit Gliick zu erklaren unternommen hat." For 

 his methods are, in principle, perfectly correct and suffi- 



