economical grounds, and also because it is a matter of experience that, with a number of glasses, the desirable properties are more easily realized when notable proportions of cullet are used. Of these two types of resistance glass (A) requires a rather higher temperature in the making and on the whole presents more difficul- ties than (B). It has also a somewhat higher coefficient of expansion, and on that ground is less liable to withstand sudden changes of temperature. Previous remarks on the influence of long-founding for securing homogeneity and stability apply in a marked manner to such a glass as (A), and this type has been made of very high chemical resistance and of satisfactory behaviour when quickly heated or cooled through a greater range of temperature than it would usually be exposed to in a laboratory. In comparison with glass (B) it is generally more reliable for working in a flame. Several examples of the type (B) tend to show reduction of zinc in a blow- pipe flame, but glasses of type (B) can be, and have been, made by British manufacturers, which exhibit none of this reduction even in a very hot flame. The general resistance of glass (B) to chemical action is good, but with hot strong solutions of caustic alkalies it does part with some zinc, and, to a very small extent, this is true of its behaviour with strong acids. Good examples of glass (A) are more resistant in the sense that they are less soluble in such reagents, but the slight action which does occur causes a roughening of the sur- face of the glass which is noticeable, while in the case of glass (B) . the surface is left polished even though the solvent action on it may have been much greater.
Balancing the evidence of the advantages and disadvantages of the two types in their general applications to laboratory work, it is probably fair to give preference, on the whole, to glass of type (B). It is inherently more capable of withstanding sudden changes of temperature, and because it is the easier glass to make, there is less likelihood with vessels made from it of mishaps due to imperfect manufacture of the glass. Whichever class of glass is chosen, all vessels and apparatus made from it need thorough annealing.
Combustion Tubing. Tubing of very hard glass is essential for many laboratory experiments, and since it is largely used in the analysis of carbon compounds by combustion it has come to be known specially in this connexion, but in tubing of various diameters it is required for a number of other pur- poses. Most of these preclude the use of any compounds of arsenic or antimony in the composition of batch mixtures for making the glass. Before the introduction of a new type of tubing from Jena, combustions and other operations at high temperature were carried out in a potassium-calcium-silicate glass, the best- known form of which was Kavalier's combustion tubing. The general composition of this glass is indicated by the following percentages to the nearest whole numbers: SiO 2 , 78; CaO, 8; K 2 O, 12; Na 2 O, 2. Glass of this kind served many useful pur- poses in laboratories, but it was difficult to use in a blowpipe flame, considerable skill being needed to work it quickly enough to avoid devitrification to an extent sufficient to roughen the surface and bring about a pasty condition which prevented the glass from flowing under heat. The Jena glass which took its place possessed greater plasticity over a longer range of temperature, and was stiff enough to allow of tubing being usable at a temperature at which the older kind tended some- what suddenly to collapse. During the war very hard glass tubing was much needed, and as the result of experiments on a laboratory scale and in glass works, tubing of a type similar to the Jena combustion tubing was produced fully equal to any obtained before the war. With regard to hardness and suitability for working in the flame it fulfils its purpose most satisfactorily. It differs advantageously in one respect from the pre-war glass, in that it does not show anything like the same tendency to become opal when heated for a long time. The following is the composition for a batch mixture, given as for other glasses in the percentage of oxides: SiO 2 , 68-5; B 2 O 3 , 5-5; A1 2 O 3 , 6; CaO, 8; BaO, 6-8; Na 2 O, 3-2; K 2 O, 2. Remarks made about formulae for batch mixtures of glasses previously mentioned apply to this formula in respect of adjustments for addition of cullet and for some modifications to suit different furnaces. With this glass, however, there is not much latitude allowable if the full hardness of the glass is to be realized and difficulties in manufacture are to be avoided. The glass is one requiring a high temperature for its successful production, and is another example of the need for such glasses calling for investigation of refractories in order to make their production possible.
Thermometer Glass. The manufacture of thermometers of
all kinds has been carried on in Great Britain for many years, and British capillary tubing of high quality and technical perfec- tion has long been available for their production. The tubing has been made both from lead glass and from various other types of glass, and has been in constant demand-. An ideal glass for thermometers, in addition to being a good durable and workable glass, must be of such a nature that bulbs blown from it are constant, in that after being heated they rapidly return to their original volume. Thermometers made from such a glass would not show any change in their zero points after use.. Jena thermometer tubing has gained a high reputation for close approximation to this ideal, and large quantities of it have been used by British thermometer makers. Mention should be made of the fact that at least one British glass manufacturer produced tubing also near to this ideal some years before the war. During the war very great numbers of thermometers were called for, the greater proportion being for medical purposes, but many also for scientific and industrial use. The production of these drew attention to the subject of glass for thermometers generally. Guided by their own knowledge and experiments, and assisted, in some instances, by other investigations, manufacturers of glass produced tubing to meet the demand, not only in lead glass, for the production of which they were ready and pre- eminent, but also in other varieties of glass having properties closely similar to two Jena glasses of high reputation. One of these can be used for thermometers, capable of standing high temperatures up to about 500 C., and the other is for more general application. The following formulae, given as for other glasses in percentages of oxides arid with similar reservations, indicate the nature of batch mixtures for these types of glass :
High Temperatures
SiO, 73-5
B 2 O 3 97
Al.,0, 5-8
Na 2 O n-o
Ordinary Temperatures
SiO 2
B 2 3
A1 2 0,
CaO
ZnO
Na 2 O
67-0 2-5 2-7 6'5 6-7
14-6
Vessels and Apparatus. If we turn from the character of the glasses themselves to the vessels and apparatus made from them, scientific glassware may be broadly classified as furnace-made and as lamp-blown. The former is for the most part produced by blowing into moulds molten glass gathered from the furnace on a blowing-iron. When the variety in shapes and sizes of flasks, beakers and other apparatus used in laboratories is considered, it will be realized how great a development had taken place in this direction after the war in a British industry in which, for several years, practically none of this type of apparatus had been made. So also in the lamp-blown apparatus had there been a remarkable extension in development. Before the war, lamp workers for laboratory apparatus were few in number in Great Britain and were chiefly engaged either in making a compara- tively small amount of apparatus to special design or in repair work. During the war numbers of workers of both sexes were trained in lamp-blowing generally, and in 1921 those making scientific glassware were producing practically all the varieties of this kind of apparatus needed in laboratories, the best ex- amples comparing favourably with any obtained in the past from abroad. Glass for such apparatus is supplied to the lamp blower in the form of tubing, in the production of which, there- fore, there had also been a great development.
The production of scientific glassware arising out of the needs of the war was one of the most noteworthy extensions of glass manufacture in Great Britain. Since so little of this kind of glassware had been made there for such a long time, the manufacturers were not, in the majority of instances, equipped with the knowledge and experience necessary to start at once. The deserved reputation of most of the scientific glassware of foreign origin made it natural at first to attack the problem of its reproduction. It is only justice to foreign manufacturers to acknowledge indebtedness to them for a number of types to work to. At the same time it would be injustice to British manu- facturers of glass to give the impression that, among the great
