522 



NA TURE 



\_Scpt. 30, 1S80 



below the critical the deviation (from Mariotte's law), first posi- 

 tive for a temperature sufficiently low, becomes nil, then nega- 

 tive, with increasing temperature ; but beyond a certain negative 

 value it diminishes indefinitely without changing sign. 6. For 

 pressures between the critical pressures and a superior limit, 

 special for each gas, the period during which the deviation is 

 positive is preceded at a lower temperature by a period in which 

 it is negative ; so that the deviation twice changes sign. 7. 

 Beyond the upper limit of pressure indicated in the preceding 

 law the deviation is always negative, whatever the temperature ; 

 it diminishes, in general, when the temperature increases, except 

 for pressures near the limit, where its variation is more 

 complicated. 



It is known that rain and other meteoric waters contain a 

 quantity of gas and saline matters which tliey find in the 

 atmosphere and carry with them. The amount varies with the 

 seasons, but may be estimated, on an average, at about 8 cc. of 

 oxygen, and 0*50 to o'6o cc. carbonic anhydride per litre, along 

 with small quantities of ammonia, nitrite, nitrate, and carbonate 

 of ammonium, organic matters, and chloride of sodium. In a 

 recent paper to the Belgian Academy M. de Koninck holds that 

 in the alteration and metamorphism of rocks by infiltration of 

 those waters may be found the solutions of many questions in 

 geology hitherto unsolved. The facts he cites relate to tertiary 

 and quaternary deposits which in many parts of Belgium are so 

 transformed by the waters in question that it is impossible 

 to recognise, them if account be taken only of petrographic 

 characters. 



From observing how different persons gave different estimates 

 of the apparent size of blood-corpuscles seen in the microscope, 

 M. Montigny was led to make a series of farther experiments on 

 the suljject (which are described in the Bulletin of the Belgian 

 Academy, No. 6). He comes to the conclusion that even for 

 good observers an estimation of the kind referred to is princi- 

 pally affected by the length of distinct vision, but that often this 

 appreciation is subject to the influence of occult causes which 

 affect the relation between sensation and judgment. The exami- 

 nation of microscopic of ijects may be influenced, lilie astronomical 

 observations, by a kind of personal error, by reason of which 

 individuals have a tendency to see microscopic images, some 

 larger, others smaller, than they should appear, abstraction 

 being made of the influence of the length of distinct vision on 

 our appreciations. These conclusions, it is pointed out, do not 

 at all affect the exactness of measurements deteniiined hy savants 

 with the microscope, but they tend to show that each observer 

 should measure for himself the different magnifying powers of 

 the instrament he uses, obtained by changes of eye-pieces and 

 objectives. 



With the view of demonstrating the mechanical action of 

 electrolysis, all action of heat being excluded, Signor Basso has 

 lately experimented thus [U Nuovo Cim,, ser. 3, torn. vii.). A 

 thin square glass plate is covered with collodion, and on this when 

 dry is put a thin layer of good gelatine, mixed « ith about -^-^ of 

 its weight of a saturated solution of bichromate of potash. The 

 bare side of the plate is exposed to light, to attach the gelatine 

 layer. Then the plate is put in an aqueous solution of chloride 

 of gold till the upper layer is impregnated v\ ith the gold salt, and it 

 is exposed to diffused daylight. Next the covered side is strewed 

 with fine graphite, and the glass connected liy means of four fine 

 wires running along its sides to the negative pole of a battery. 

 The plate is then placed in an ordinary bath of sulphate of 

 copper. The copper is deposited regularly on the whole of it. 

 In a few days wrinkles and bubbles appear; and if the copper 

 have been deposited as far as the borders, the plate may at length 

 even break, thus proving the mechanical force, which is a direct 

 consequence of electrostriction. 



ON THE PRESENT STATE OF SPECTRUM 



ANALYSIS'^ 

 AT the Sheffield meeting of the British Association a committee 

 was appointed to report on the present state of spectrum 

 analysis. The committee has this year presented its first report. 

 The report is divided into four parts : — 



1. On the spectra of metalloids, drawn up by Dr. A. Schuster. 



2. On the influence of tenqierature and pressure on the spectra 

 of gases, drawn np by Dr. A. Schuster. 



. ' Aljstract of Report read at tlie Su-.-insea meeting of the British Asso. 



3. On the emission spectra of the rays of high refrangibilily, 

 drawn up by Prof. W. N. Hartley. 



4. On the absorption spectra of the rays of high refrangibility, 

 drawn up by Prof. A. K. Huntington. 



In the report on the spectra of metalloids, we have for each 

 clement a full account of the literature on the subject with all 

 necessary references. The various spectra of each matalfoid and 

 its compounds are enumerated, and special stress is laid on the 

 discussion which nearly always has taken place on the chemical 

 origin of these spectra. It will be found that often more work 

 is needed to clear up doubtful points, but there is no special 

 controversy at issue at the present moment except in the case of 

 the carbon spectra. A discussion of very long standing is still 

 occupying the minds of many spectroscopists as to whether the 

 spectrum which is seen at the base of every candle flame is due 

 to carbon or to a hydrocarbon. The arguments and experiments 

 on either side are given in detail and are finally summed up as 

 follows: — " Those who believe the spectrum to be due to th; 

 element carbon rely chiefly on the brilliancy with which these 

 bands are developed ^^hen cyanogen is burnt in oxygen, also 

 when the spark is taken in cyanogen, carbon tetrachloride, and 

 carbonic oxide at high pressure ; all the gases being dried with 

 the greatest care. Those who oppose this view and who hold 

 that the spectrum is due to a hydrocarbon refer to the impossi- 

 bility of excluding all traces of moisture, and to the fact that this 

 spectrum is well developed under circumstances \\ here we know 

 hydrocarbons to be present." 



When cyanogen is burnt a series of bands appears in the blue 

 violet and ultra-violet, and another controversy has taken place 

 whether these bands are due to carbon or to a compound of 

 carbon and nitrogen. Two papers have lately appeared on the 

 subject. One by Mr. Lockyer, in which he describes an experi- 

 ment in which the bands were seen in a spark taken in carbon 

 tetrachloride, although the nitrogen lines were not visible in the 

 jar discharge ; and another by Professors Liveing and Dewar, in 

 which these bands were traced to impurities of nitrogen in all 

 cases in which they were seen. A spark in carbonic oxide 

 showed the bands when the gas was prepared from ferrocyanide 

 of potassium, but not when it was made by heating a mixture of 

 quicklime with pure and dry potassium oxalate. When all the 

 air had been properly expelled a tube containing carbon tetra- 

 chloride did not show the bands. 



The following quotation will give an idea of the points which 

 are discussed in the second report : — 



" We shall endeavour for clearness' sake to arrange our mate- 

 rial under five different hea^ls. We shall fir^t discuss what 

 changes we have a right to expect in[the appearance of a spectrum, 

 if the quantity of luminous matter is increased or if the tempera- 

 ture is raised, the absorbing properties of the gas remaining 

 unaltered. We shall next speak of the widening of lines, which, 

 as we shall see, often accompanies an increase of pressure. Then 

 we shall treit of the different spectra given by one and the same 

 body at different temperatures ; and we shall see how far satis- 

 factory explanations have been offered for their existence. 



"So far our road will be clear ; but we shall see that these 

 spectra of different orders, as they have been called, are only 

 extreme cases of continuous changes which are nearly always 

 going on. Very often we can refer these continuous changes to 

 a gr.adual displacement of one spectrum by another ; but often 

 we shall not be alile to prove the existence of a second spectrum. 

 There is «/;7D/7 nothing impossible or even improbable in the 

 view that the relative intensity of different lines may be different 

 at different temperatures, and often \\ hen we observe a variation 

 \\e may equally well explain it by assuming the gradual appear- 

 ance of a new spectrum or an alteration only in the relative 

 intensities of the lines. It becomes then a matter of extreme 

 difficulty to decide which of the two suppositions is correct. In 

 doubtful cases vi e may often be able to obtain important infor- 

 mation by means of a method which is little understood even by 

 siiectroscopists. It is the method which has first been extensively 

 used and investigated by Mr. Lockyer of projecting an image of 

 the luminous source, spark, arc, or flame on the slit of the spec- 

 troscope and thus localising the spectra, which are thrown and 

 confused together if the luminous source is examined directly 

 without the interposition of a lens. We shall see how by means 

 of this method we shall often at a single glance be able to tell 

 how the body will behave at different temperatures and under 

 different pressures. Many facts which have been quoted as 

 remarkable might have been foretold by means of this method. 

 Our fourth chapter will be devoted to it. In our last chapter we 



