number of varieties which they had to make during the war for all kinds of purposes, there were not very many which were produced de now, as the result of the work of a number of investigations outside, as well as inside, the industry, and of ready enterprise on the part of the manufacturers. For several of the glasses needed there were no available data to go upon, and the knowledge and experience required for guidance in the earlier stages of their production had to be gained by research.
The best examples of glasses for scientific purposes, of British manufacture, are now fully equal to any pre-war glass, and some are superior. The glasses already mentioned are the chief ones required for the production of laboratory apparatus, but they do not, by a long way, exhaust the list of glasses called for during the war for special scientific use or for industrial purposes. Examples are here briefly referred to.
For glass for minors' lamps, a glass withstanding rapid changes of temperature exceptionally well was necessary, since the lamp glasses are thick and the flame of the lamp niay often touch them. There was an urgent demand for them early in the war. It was suc- cessfully met, and such glasses of British make are now produced in large quantities. Another glass on similar lines, but differing somewhat in composition, was prepared for the production of chim- neys for incandescent and high-pressure gas illumination, paraffin lamps, etc. In addition to withstanding heat changes well such a glass must be markedly resistant to the chemical action of hot prod- ucts of combustion. Both these glasses consist chiefly of alkaline boro-silicates having a high percentage of boric anhydride. They need a high temperature for their successful production in a homo- geneous state. When well founded their low coefficients of ex- pansion render articles made from them highly resistant to sudden variations in temperature over a long range.
Glass rods for half-watt electric lamps were required, to hold the thicker tungsten wires which support the filament of this metal. They had to be made specially, since no existing glass of British make capable of withstanding heat changes was also sufficiently reliable in respect of not cracking round the sealed-in wires. This glass in most cases involved also the production of special rods to join with it and with the stem of the lamp.
Other glasses were needed which, while making safe joints with ordinary laboratory tubing, etc., would hold platinum, copper, iron or nickel wires. Such glasses are often described as sealing-in enamels. Several of these have been made, and, generally speaking, they are of the type either of a soft glass containing a high percentage of lead, or of one free from lead and containing a notable proportion of a fluoride, such as cryolite. The coefficient of expansion of the glass, in relation to that of the metal wire used, has to be taken into account, but it is not the only factor, as may be just indicated here by the mention of a sealing-in enamel which is successful with platinum and copper but cracks with iron, nickel or tungsten.
Other glasses, and glassware from them, which had to be made during the war will be mentioned very briefly. They were of great importance, but, generally speaking, they were familiar to British manufacturers, and their manufacture did not need the extensive preliminary investigations and trials which the production of most of the foregoing glasses involved.
Bulbs for Making Ordinary Electric Lamps. Before the war some- what less than a quarter of our requirements of these was made by British manufacture. A very great extension of this part of the industry during the war was urgent. In 1918 bulbs were being made at the rate of about 1,000,000 per week.
Jars for Preserving Fruit and Meat. Though numbers of these had long been made in Great Britain, about 80% of the total number used had been obtained from abroad. Great increase in the pro- duction of these vessels was required to meet the needs, enhanced as they were by the war. Bottles for a great variety of purposes had always been made by British manufacturers, but not in the great quantities which were required when sources of supplies from abroad were cut off or were inadequate. The extension of this part of the glass industry was very great even on the older lines of manufacture, but the necessity for more economic production led to a review of methods and to the adoption of new machinery.
Glassware for Medical Purposes. Some of this has for many years been made in Great Britain, but not in sufficient quantities to supply the demand, and much of it was obtained from abroad. The war caused a great increase in the demand, and very large quantities of vials, tubes, syringes, graduated measures, etc., had to be made. Most of these could be produced from glass, and by methods familiar to manufacturers, but some requirements had to be met by investigation and experiment before suitable glass was produced. In connexion with medical glassware, artificial human eyes may be mentioned. For their production there are required opal glasses to suit variations in the tint of the sclerotic; bright clear glass for the lens; black glass for the pupil, and a great variety of coloured glasses for the iris; a clear glass containing fine embedded
threads of opal used for imitating the irregularly radiated appearance of the iris, and a red glass for the veins of the eye. Artificial eyes had for many years been made in Great Britain, but many were im- ported. Most British makers of them are used to working in lead glasses, and many of their products will bear comparison with the best of foreign origin, which, as a rule, are made from glasses free from lead. Experiments for the production of such glasses as the latter furnished the data for their manufacture.
There was considerable increase during the war in the production of coloured glasses, e.g. for spectacles to protect the eyes of the great numbers of men working at steel furnaces. Coloured glasses in considerable variety were also wanted for other purposes, but in comparatively small amounts. Some of them needed investigation and a number of experiments before the conditions for their pro- duction could be determined.
From what has been stated already it may be gathered that a great advance had to be made in glass manufacture through needs arising out of the war, and sufficient has, perhaps, been said to indicate that the knowledge and experience gained in meeting them had placed the British glass industry in this respect in 1921 in a very different position from that of 1911.
Optical Glass. Of none of the glasses already mentioned can it be said there was more imperative need for their production than for the variety of glasses required to make the numerous optical instruments used during the war. Early in it there was no doubt that the supplies of optical glass existing in England would soon be exhausted. For about three quarters of a century, Messrs. Chance Brothers of Birmingham had produced optical glass. They were enabled greatly to extend their facilities for production, in order to meet the demands which rapidly arose and were very urgent. The change which was brought about in the production of optical glass in England will be gathered from the following comparative figures. For a year or two before the war, out of the total amount of optical glass used in Great Britain, approximately 60% was imported from Germany, 30% from France, and 10% was of English manufacture. In 1916 81% was English and 19% was obtained from France, while the total quantity supplied by Messrs. Chance Brothers was about 1 8 times as much as they sent out in 1913 and over three times as much as the total quantity of optical glass from all sources used in Great Britain in that year. About the middle of 1917 the Derby Crown Glass Co., which was formed in the autumn of 1916 for the manufacture of optical glass, was sup- plying it. Figures for the first quarter of 1918 show that 96% of the optical glass used in Great Britain was made at home, while France supplied only 4% of the British requirements. In that first quarter the output of optical glass made in England was about nine times as much as the total quantity of English manufacture produced in the whole of 1913, and it was being made at the rate of an annual production of more than six times the total quantity of optical glass from all sources used by British optical instrument manufacturers in 1913. This great increase in production was due entirely to the war, since during it very little optical glass was used for purposes other than the manufacture of instruments for the fighting services. The compulsory extension of manufacture called mostly for development in quantity production rather than increase in the number of types manufactured. A few types not hitherto made in England have been produced; but Messrs. Chance Brothers for some years have manufactured a number of glasses having properties similar to several types of Jena optical glass. Both this firm and the Derby Crown Glass Co. have been called upon for glasses having pre-determined optical constants, and the meeting of these demands has involved a considerable amount of investigation and experiment. No completely new type of optical glass has been manufactured; but in some instances the requirements of the optician have necessitated a departure so marked as to constitute an extreme variety very like one.
It is not easy to suggest a strict definition of " type " as applied to optical glass. The two types of earlier days were " flint " and " crown " the former containing lead oxide and the latter calcium oxide along with alkalies and silica. The names are convenient, as their connotations are understood and they have become conventional; but a glass free from lead may be
