Vol. XXII. No. 8.] 



POPULAE SCIENCE NEWS. 



117 



Practical C&emtetrp anlf tiit ^tts. 



GLASS-MAKING. 

 The first discoverer of glass is unknown. 

 The familiar story of the Phoenician sailors 

 building a fire upon a sandy beach, under a 

 kettle supported by blocks of soda, has no 

 foundation in fact ; and, moreover, glass beads 

 have been discovered in prehistoric tombs of 

 unknown anliquit}". 



Chemically, glass is a silicate, or a compound 

 of silicic acid and various bases. It is formed 

 by fusing silicic acid (common sand) with the 

 carbonates of the alkalies, and in some cases 

 with metallic oxides. Ordinary window-glass 

 is a silicate of lime and soda. Plate glass 

 contains, in addition, some silicate of potas- 

 sium. Crown glass is now rarely used, except 

 for optical purposes, and is a silicate of po- 

 tassium and lime.- Flint glass, which is com- 

 monly used for table-ware and. ornamental 

 purposes, is a silicate of potassium and lead. 

 It is made by fusing three hundred parts of 

 the purest white sand, two hundred parts of 

 red oxide of lead, one hundred parts refined 

 pearlash, and thirty parts of nitre. The effect 

 of the lead is to give increased brilliancy, and 

 renders it soft and easily cut. The nitre is 

 added to oxidize any accidental impurities, 

 which might otherwise reduce the lead to the 

 metallic state. 



Bottles and similar coarse ware arc made 

 from impure materials, and the dark color is 

 due to the presence of iron. Verj- few sands 

 are entirely free from iron, but a mere trace is 

 sufficient to injure the color of the glass. The 

 best sand in the world for glass-making is 

 found in fSerkshire County, Mass., and is sent 

 to all parts of the country. 



The materials are mixed together, and fused 

 in pots or crucibles of burnt clay. These pots 

 are very likely to break when submitted to the 

 heat of the melting-furnace, and must be pre- 

 pared with the greatest care, only the ver}- 

 l)est quality of clay being used. A compar- 

 atively low heat is applied at first, so as to 

 drive oflT the moisture and carbonic acid, and 

 cause the materials to partially melt together. 

 It is then increased until the whole mass is in 

 a state of fusion. The various impurities rise 

 to the top, forming a scum known as "glass 

 gall," which is removed with iron shovels. 



After the materials are entirely' melted, the 

 heat is continued for some time, so that the 

 glass is kept in a thinly fluid condition. This 

 allows the impurities to settle to the bottom 

 of the pot, and the glass to become clear and 

 uniform in quality. The temperature is then 

 lowered to about 1500° F., at which point the 

 glass loses its fluid consistency, and becomes 

 pasty, and fit for blowing or working into 

 sliape. 



The tools of the glass-blower are of the 

 simplest i^ossible description. The principal 

 one is the blowpipe, which is merelj- a tube 

 of iron. The glass-blower takes a sufficient 

 (juantity of melted glass upon the end of this 

 tube, and by blowing into it with his breath, 

 and various dexterous manipulations, in a few 



moments transforms it into a beautiful goblet 

 or other useful or ornamental article. Glass- 

 blowing is almost altogether a matter of 

 personal skill and dexterity, and a skilful 

 glass-blower commands high wages ; but the 

 work is very exhausting, and liable to produce 

 diseases of the lungs. 



After the glass is blown and cooled, it re- 

 mains in a state of internal tension ; that is, 

 owing to the rapid and uneven cooling, the 

 glass is strained in diflferent directions, ren- 

 dering it very liable to fracture. Such a strain 

 was formed in a piece of glass for one of the 

 lenses of the great Lick telescope, and de- 

 tected by the aid of polarized light. It was, 

 however, determined to proceed with the cut- 

 ting, with the result that the glass split into 

 three pieces during the process. To relieve 

 these strains, and allow the molecules to be- 

 come better acquainted with each other, the 

 articles of glass are placed in a long annealing 

 furnace, which is kept very hot at one end, and 

 cool at the other. The glass is slowly moved 

 through it by machinerj', taking a daj' or more 

 to complete the journey', and, by this gradual 

 cooling, is rendered fit for use. 



Ordinary "German " window-glass is made 

 by being blown out into a cylinder which is 

 then split and rolled out flat. Plate glass 

 is cast by flowing it over a metal table, and 

 is afterwards ground flat, and polished. The 

 bronze casting-table in use at a factory in 

 France cost twenty thousand dollars. In the 

 process of grinding and polishing, the glass 

 sometimes loses half its original weight and 

 thickness. 



Cut glass is a fine variety of flint (lead) glass ; 

 and the " cutting," — which is really a grind- 

 ing process, — is done on wheels, and requires 

 the greatest skill and experience. Pressed 

 glass is an imitation of cut glass, and is made 

 by pressing the melted glass into a mould. 

 Some of the finest pressed work is almost 

 equal to that cut by hand. 



Soluble or water-glass is an alkaline sili- 

 cate, containing a large proportion of the 

 alkali. It is readily soluble in water, and is 

 largely' used in the arts. A glass containing 

 a large proportion of lead as a base is known 

 as paste, and used to imitate precious stones. 

 The coloring of glass was described in the 

 Ma}' number. 



Glass for optical instruments must be made 

 with the greatest care ; and it is difficult to 

 obtain it free from bubbles, spots, and layers 

 of unequal density. On account of tlie greater 

 refractive power, verj- heavj' glass is desirable 

 for lenses and prisms. This is obtained by 

 the use of a large percentage of lead. Thal- 

 lium, molybdenum, boron, and tungsten have 

 also been used to a limited extent ; and a sil- 

 icate of lead made by Faraday many years 

 ago possessed so high a refractive power, that 

 it became quite celebrated. Professor T^-n- 

 dall, in his lectures in this countrj', made use 

 of some of Faraday's glass. 



Few things have contributed more to the 

 comfort of mankind, both directl}- and indi- 

 rectly, than glass ; but it is only in compara- 



tively modern times that it has l)een produced 

 so cheaply as to come into universal use. For 

 many hundred years it was an article of lux- 

 ury only, and glass windows are specially 

 taxed in some European countries to the pres- 

 ent day. While the cost of manufacture will 

 probably never be much less, the qualitj^ and 

 beauty of glassware is constantly improving ; 

 and the poorest person may now use on his 

 table, every day, glassware more beautiful than 

 a king could obtain not many years ago. 



MAGIC MIRRORS. 

 There are occasionallj' brought to this 

 country, 'from Japan, mirrors of tliin polished 

 metal, on the back of whicli are figures and 

 designs cast in relief. Some of these are 

 known as "magic mirrors," from the singular 

 fact, that, when the sunlight is reflected from 

 one of tliese mirrors upon a screen or wall, 

 the design on tlie back is reproduced in the 

 reflection, apparentlj- as if the metal was 

 transparent. The cause of this remarkable 

 effect has been unknown until quite recently, 

 when Mr. F. E. Ives discovered the true 

 explanation. It seems that, after casting the 

 mirrors, the front surface is polished by being 

 held by the back against a revolving wheel. 

 As the polisher presses the mirror against it, 

 the thinner portions spring back, or " give," a 

 little, while the thicker parts formed by the 

 raised figures do not yield as much, and con- 

 sequently are ground down a little deeper. 

 This difference in the surface, while not 

 perceptible to the eye, is sufficient to make a 

 difference in the reflective power ; and thus a 

 shadow of the pattern on the back is reflected 

 from the front surface. The correctness of 

 this explanation is shown by the fact that in 

 mirrors where the figures on the back are 

 small and close together, the " magical." effect 

 is not produced ; the thin parts of the metal 

 corresponding to tlie open part of the pattern 

 not being large enough to spring back to any 

 extent under the pressure. 



A similar effect can sometimes be observed 

 with the old-fashioned bright brass buttons, 

 which were cast and polished like the Japanese 

 mirrors. By reflecting the light from their 

 front surface upon a piece of white paper, the 

 letters on the back will appear in the reflected 

 image. 



METALLIC ALLOYS. 



A RECENT lecture at the Royal Institution was 

 by Mr. W. C. Roberts-Austen on the properties 

 of certain alloys. 



The lecturer began by speaking of the changes 

 in the molecular state of bodies sometimes set up 

 by very small causes, and he exhibited a warm 

 basin painted inside with a saturated solution of 

 platino-cyanide of magnesium. The bowl appeared 

 to be warm and empty until he breathed into it, when 

 it became of a crimson color, in consequence of the 

 traces of moisture taken up by the salt from his 

 breath ; when he again warmed the bowl, to drive 

 off the traces of moisture, the coating became once 

 more colorless. Metallic tin, he said, will readily 

 bend; but let it be alloyed with but a small propor- 

 tion of arsenic, it becomes so brittle that a small bar 



