Jl'NE. 1915. 



KNOWLEDGE, 



183 



Mr. Nelson quoted an extract from Messrs. SoUitt and 

 Harrison's paper, read before the British Association at 

 Hull in 1853. 



" We in Hull first discovered the delicate marking on 

 their silicious coverings, and pointed them out to others as 

 the proper tests for lenses. The first of the Diatomaceae 

 on which the lines were seen was the Navicida hippocampus 

 of Ehrenberg. . . . This was early in 1841, when specimens 

 were sent to the Microscopical Society of London. . . . 

 Also to Mr. Smith, Mr. Ross, Messrs. Powell & Lealand, 

 M. Nachet in Paris, and Professor Bailey in America, all 

 of whom at once saw the excellence of these objects as tests 

 for the microscope." 



The next conversational meeting for the exhibition of 

 objects and for discussion will be held on Tuesday, June 8th, 

 at seven p.m., and the five hundred and ninth ordinary 

 meeting on Tuesday, June 22nd, at eight p.m., both at 20, 

 Hanover Square, W. Gentlemen wishing to attend either 

 of these meetings as visitors are invited to apply for cards 

 of admission to the Hon. Secretary, Mr. J. Burton, 8, Somali 

 Road, West Hampstead, N.W., or to any of the leading 

 opticians. 



PHOTOGRAPHY. 



By Edgar Senior. 



POTASSIUM METABISULPHITE.— Of all the com- 

 binations of sulphurous acid with alkalies, that prepared by 

 treating potassium carbonate with the acid until the quan- 

 tity of the latter corresponds to the bisulphate, when the 

 substance known by the name of metabisulphite is de- 

 posited in the form of fine crystals containing no water of 

 crystallisation, appears to be the best so far as keeping 

 properties are concerned. Potassium metabisulphite 

 (KjSoOj) is an acid salt, and contains a very much larger 

 quantity of sulphurous acid than sodium sulphite, and keeps 

 much better in the solid state than the latter; moreover, 

 having no water of crystallisation, its composition is not 

 rendered uncertain. The quantity (theoretically) of 

 sulphurous acid is 57-65 per cent., and a sample kept for 

 two years in a corked bottle was found on analysis to contain 

 55-05 per cent, of acid (Namias). Metabisulphite, being 

 acid, is, like most acids, a good preservative of pyrogallol 

 in solution, and may be employed in developing solutions 

 by using about one-third the quantity that would be required 

 were sodium sulphite used instead. As the acidity of the 

 substance neutralises a part of the alkali added to the 

 developer, this must be taken into account, as a portion 

 of the alkali would thus be rendered inoperative. It might 

 be thought that all that was necessary would be to add^ 

 further quantity of alkali, such as soda carbonate, to make 

 up for that lost ; but iit most cases such a course is not to be 

 recommended, since the carbonic acid which is set free 

 would give ri.se to the formation of bicarbonate of soda, 

 and this retards the action of the developer, especially in 

 the case of hydroquinone. On this account the acidity 

 of the metabisulphite should be neutralised with a little 

 caustic soda or potash before the alkali required for de- 

 velopment is added. In the case of developers, however, 

 such as metol or edinol, which are more energetic in their 

 action, it is unnecessary to use caustic alkali, as a slight 

 increase in the quantity of alkaline carbonate suffices. 

 Although metabisulphite keeps so well in the dry condition, 

 it does not do so in solution, unless the bottle is full and 

 well corked, when the sulphurous acid liberated appears 

 to protect the surface of the liquid from direct contact with 

 air, and under such conditions the solution will keep 

 better than sulphite does ; otherwise there is little differ- 

 ence in the keeping properties of either. The other bi- 

 sulphite, sodium bisulphite, contains (theoretically) a little 

 more sulphurous acid than potassium metabisulphite ; 

 but as it is much less stable, losing its excess of sulphurous 

 acid much more readily than the mctabisulpliitc does, it 

 usually contains less, and at the same time has a tendency 

 to become converted into the anhydrous sulphite having 

 the properties already mentioned. 



SULPHITES AND METABISULPHITES.— The ex- 

 tensive use that is made of sulphites and metabisulphites 

 in photography suggests the question of their relative 

 merits and stability. The sulphite that is most commonly 

 employed in photographic work is the crystaUised sodium 

 sulphite, which is represented by the formula Na,SO, . 7HjO. 

 By exposure to air, either in the solid form or in solution, 

 oxygen is absorbed and sodium sulphate is formed; in fact, 

 it appears impossible to obtain soldium sulphite absolutely 

 pure, as analysis has shown that the purest sample did not 

 contain more than ninety per cent, of sulphite, while it 

 averaged from sixty to seventy per cent, in good samples, 

 and in the case of inferior ones from ten to thirty per cent. 

 (Namias). In the case of the anhydrous sodium sulphite 

 the quantity of sulphate present always appears to be 

 considerable, a result brought about in all probability by 

 the process of heating employed to remove the water of 

 crystallisation. Some samples of the anhydrous salt con- 

 tain only about fifty-five per cent, of sodium sulphite. 

 Inferior samples of sodium sulphite may also contain 

 sodium carbonate as an impurity, as the sulphites are pre- 

 pared by saturating solutions of the carbonate with sul- 

 phurous acid. In order to test for the presence of carbonate 

 of soda in a sulphite, the gaseous mixture of CO^ and SO,, 

 obtained by the action of dilute sulphuric on hydrochloric 

 acid, may be first passed into a solution of bichromate on a 

 cupric or ferric salt, and, finally, through Ume or bar%'ta- 

 water. The indicator kno\vn as phenol-phthalein may also 

 be employed, as this reagent is turned red when added to 

 the solution of sulphite if the carbonate is present. The 

 presence of sulphate is shown by the addition of barium 

 chloride to an acidulated solution of the sulphite, a pre- 

 cipitate of barium sulphate being formed. Tliis reaction 

 may therefore be made use of as a means for estimating the 

 quantity of sulphate present in a given quantity of sulphite. 

 It will now be readily seen from what has been stated that 

 in making up solutions containing sodium sulphite the 

 actual quantity of the sulphite present may be very con- 

 siderably less than the weight of salt taken as such leads 

 us to suppose, while there may be a great deal of sulphate, 

 a body which is not only undesirable, but in many cases 

 harmful. WTiile the anhydrous sodium sulphite from its 

 powdered form is more convenient, it is not to be recom- 

 mended in practice, as it readily absorbs water, when the 

 heat of hydration faciUtates its oxidation, the minute 

 grains of the anhydrous salt exposing a far greater extent 

 of surface to be acted upon, weight for weight, than is the 

 case with the crystallised substance. Thus crvstalhsed 

 sodium sulphite does not require such careful preser\-ation 

 from contact with the air as the anhydrous salt does. One 

 of the simplest ways to keep sodium sulphite in the solid 

 state is to preserve it in bottles from which the air has been 

 displaced by means of a stream of ordinary house gas 

 obtained from a burner. As for sodium sulphite in a state 

 of solution, it appears to be far less stable than in the solid 

 condition ; hence keeping it in this condition is not 

 advisable. 



PHYSICS. 



By J. H. Vincent, M.A., D.Sc, A.R.C.Sc. 



THE DISCOVERER OF INVAR.—Science announces 

 that the city of Philadelphia, acting on the recommendation 

 of the Franklin Institute, Philadelphia, Pa., has awarded 

 tlie John Scott legacy medal and premium to Dr. Charles 

 Edward Guillaume, of Sevres, France, for his alloy in\-ar. 

 This alloy, which is approximately a thirty-six per cent, 

 nickel steel, is remarkable for its small rate of change of 

 size with alteration of temperature. Some specimens 

 have been prepared with a temperature coefhcient sensibly 

 zero, while others have very small coefficients, which may 

 be either positive or negative. The use of this alloy in 

 clock-making has enabled attention to be paid to small 

 corrections which had pre\'iously been swamped by the 

 outlying irregularities left uncorrected in the older methods 

 of temperature compensation. In the measurement of the 



