August, 1913. 



KNOWLEDGE. 



313 



use in the production of these tones have been given, a' though 

 the following is perhaps as satisfactory as any : — 



Mercuric Chloride ... 

 Potassium Oxalate... 

 Potassium Phosphate 

 Water 



3 grains 

 48 „ 

 48 „ 



1 ounce 



The temperature at which the prints are developed ranges 

 from 65 3 to 170° F., according as a warm black or one 

 approaching sepia is desired. When warm tones are obtained 

 the gradation is usually much softer than that of a black 

 print from the same negative ; there is therefore a tendency 

 to general flatness in some cases. When this is the case the 

 results may often be improved by the addition of about five 

 minims of a two per cent, solution of potassium bichromate to 

 each ounce of developer. As special paper is, however, made 

 by the Platinotype Company and others for obtaining sepia 

 prints, this should be employed, taking special care to protect 

 the same during examination of the printing from the action of 

 weak light as much as possible ; and as these prints, unlike 

 the black one, are likely to be affected by light when in the 

 acid baths a more subdued light should be employed during 

 this stage of the operation as well. 



SECTOR SHUTTERS.— We have received a copy of a 

 paper read at the Optical Convention on June 20th, 1912, by 

 Cyril F. Lan-Davis, F.R.P.S.,on the subject of sector shutters. 

 The author, in dealing with the number of leaves 

 comprised in shutters of this type, and showing that the 

 position of the pivot does not affect the ratio of the 

 total diameter of the shutter to its opening diameter, 

 proceeds to discuss the effect that the shape of the leaves, as 

 well as their number, has on the shutters' efficiency, showing 

 by means of diagrams that an alteration in the form of the 

 leaves as well as a reduction in their number results in an 

 additional area being uncovered for equal partial central 

 openings in the shutter. In doing this comparison is drawn 

 between a ten-leaved shutter of the usual iris form, which 

 opens in an expanding circle from its centre to edge (so that 

 the amount of light transmitted by the margin is very small 

 indeed), and one having two leaves only, which opens in a 

 broad band that rapidly expands to the full circle. A shutter 

 of this kind would also possess the further advantage that 

 it could be made smaller, as the ratio of the total diameter 

 to the aperture is only 2-1. Although shutters of this type 

 are not yet on the market, we believe Messrs. J. H. Dallmeyer, 

 Ltd., are making arrangements for their manufacture. We 

 have received from Messrs. Wratten & Wainwright, Ltd., 

 several of their booklets dealing with the subject of 

 orthochromatic photography and the use of light filters for 

 special purposes, such as photo micrography, and in the 

 illumination of the dark room. We may mention in con- 

 nection with our notes in the June issue, dealing with the 

 testing of dark-room light filters, that the firm make quite a 

 number of special screens for this purpose, which are 

 scientifically tested, and may therefore be relied upon to 

 afford a perfectly safe light for use during the manipulation of 

 the particular kind of plate " or sensitive surface " that they 

 are supplied for use with. While reserving any further remarks 

 for some future occasion, we may say that the name of 

 Wratten & Wainwright is always a sufficient guarantee of 

 the excellence of any product issued by them. 



PHYSICS. 



By Alfred C. Egerton, B.Sc. 



Xs? — Professor Sir J. J. Thomson, O.M., gave the Bakerian 

 Lecture to the Royal Society on May 22nd. In the course of 

 an account of his recent experiments, he surmised that a gas 

 exists, which he terms, somewhat mysteriously, Xa. When 

 the positive rays are allowed to stream back through the hole 

 in a hollow cylindrical cathode in a vacuum tube, through 

 which an electric discharge is passed, they ionise the gas 

 through which they pass. The rays produce a number of 

 differently electrified gas particles ; they give rise to : — 



(i) Atoms with one unit positive charge of electricity, 

 (ii) Molecules with one unit positive charge, 

 (iii) Multiple charged atoms, 

 (iv) Atoms with one negative charge, 

 (v) Molecules with one negative charge, 



and when the discharge is passed through air at very low 

 pressure, since oxygen, nitrogen, argon, carbon monoxide and 

 carbon dioxide, and hydrogen (from water vapour) are present, 

 and each of them may be split up in the above ways, the 

 character of the gas evidently becomes somewhat complicated. 

 Nevertheless, by submitting the gas to magnetic and electric 

 fields, the charged particles can be identified from the position 

 they occupy on a suitably placed screen or photographic 

 plate, when fields of known strength are applied ; their 

 mass can then be calculated. By such means there is found 

 in the gas of tubes in which cathode rays are allowed to 

 bombard against solids, particles which consist either of 

 carbon with four charges of electricity or a substance of 

 atomic weight 3 with a single charge — this latter is the 

 substance to which Sir J. J. Thomson assigns the name X 8 , 

 because it has not itself been isolated yet in the free state, 

 and because it should have atomic weight 3. The reasons 

 given f:>r considering that it is not carbon with four charges 

 are that it can pass over red-hot copper oxide and then over 

 potash without being absorbed, it is not changed when sparked 

 with excess of oxygen, it can pass over metallic sodium, it is 

 not condensed by liquid air, but is absorbed by charcoal 

 cooled with liquid air, while it combines with mercury vapour 

 in presence of the electric discharge, and also to some extent 

 with red-hot copper. 



It will be very interesting if this gas can be caught and 

 examined ; a new gas a little heavier than hydrogen and 

 lighter than helium may explain much that is not yet under- 

 stood. 



NOMENCLATURE OF RADIOACTIVE 

 SUBSTANCES. — A little while back an international 

 committee assembled in order to discuss matters connected 

 with radioactivity ; one question was the standardisation of 

 radioactive substances, and in order to be able to refer such 

 substances to a definite standard it was settled that Mme. 

 Curie should prepare a solution containing a known amount 

 of pure radium, the unit of radioactivity so obtained being 

 called a " curie." Another question was the nomenclature of 

 radioactive substances ; there are three main groups of radio- 

 active substances — the radium, the thorium, and the actinium 

 series. Hitherto, as the separate products of these three 

 series have been discovered, it has been usual to distinguish 

 them by consecutive letters of the alphabet, but sometimes 

 when such names have been settled a product is subsequently 

 found which lies between two such consecutive letters, e.g., 

 thorium Ci and C a . The question of the nomenclature of 

 these many products was therefore postponed till such a time 

 as it should become certain that no more intermediate products 

 are present to be discovered. However, it is unsatisfactory that 

 the matter was left in so chaotic a state as it is at present ; 

 for in different journals, or indeed often in the same paper, the 

 same product is given more than one name. 



There are two ways open — either each product should be 

 given a definite name as is the custom is for the ordinary 

 elements, or some elastic system should be devised which 

 would not only allow for the discovery of new products, but 

 would also give an idea of the properties of the separate 

 products. The former system has been favoured by Sir 

 William Ramsay in naming radium emanation "niton," 

 Professor N. Campbell has advocated the other way. 



In chemistry organic compounds are named in such a way 

 that the chemist can tell from the name of the substance 

 many of its properties, and also its relationships with other 

 substances; for instance, o naphthylamine signifies that the 

 substance contains an NH 2 group in a particular position in 

 the naphthalene molecule. It should be possible to devise a 

 system of nomenclature of radioactive substances from which 

 it would be possible to tell the main properties of the 

 substance. The nature of the rays should be prefixed to the 



