_Copying a Scale._--When a scale has been prepared on paper and it is necessary to copy that scale on the waxed-glass surface for etching, a convenient method is to employ a long wooden bar having a sharp needle passing through it at either end. The scale and object to be marked are fastened in line with one another, and the caliper bar is used from step to step. The mark is made by moving the bar through a minute portion of a circle, which provided that the bar is two or three feet in length, will not introduce any perceptible error in a scale of say a quarter of an inch in width. The arrangement is shown by Fig. 15. [Illustration: Fig. 15] _Graduating a Thermometer._--Assuming that the thermometer has been made of carefully selected tubing in which the bore is parallel and free from any small irregularities, we have only to fix the freezing point and boiling point. The intervening space may then be divided into 100 (if the thermometer is to be Centigrade) or 180 (if Fahrenheit). This division may be carried out by the method given under "Subdivisions of Graduations." A thermometer should not be calibrated until some weeks after making, as the glass bulb tends to contract. _Joining Glass and Metal._--It sometimes happens that one needs to make a more permanent and less flexible joint between a glass and metal tube than can be obtained by means of a rubber tube. To this end, any one of three slightly different methods may be employed. In the method of Chatelier one first coats the glass with platinum or silver, which may be done by moistening the glass with platinum chloride or silver nitrate and then heating to redness; a layer of copper is then deposited electrolytically on the treated surface of the glass, and soldering is carried out in the usual manner. McKelvy and Taylor call attention to two other methods in the _Journal of the Chemical Society_ for September, 1920. In one of these methods the glass is coated with platinum by covering it with a suspension of platinum chloride in oil of lavender and heating until the oil is burnt off. The metal tube is then tinned on its inner side and soldered to the prepared glass, slightly acid zinc chloride being used as a flux. In the second method, a joint is made by means of the Kraus flux, which consists of equal weights of zinc oxide, borax, and powdered soda-glass fused together. This is coated on the inner surface of the metal tube, and the hot glass tube, which has had the end slightly flanged to give support, is inserted. Fusion of the flux is completed by heating the outside of metal tube. _Silvering Glass._--In all cases where it is intended to deposit a silver mirror on a glass surface, thorough cleaning is essential. Prolonged soaking in a hot solution of potassium bichromate which has been acidified with sulphuric acid will often prove useful. The glass should then be washed thoroughly, rinsed in distilled water, and the solution should then be used.