. Practical methods of organic chemistry . esize of a pin-head, to prevent itfrom becoming dark in colour. Thesubstance is placed in a small nar-row tube (melting-point tube), madein the following way: A glass tube4-5 mm. wide is heated at one pointwhile constantly turned, in a small,blast-lamp flame, until it becomessoft, and is then drawn out from—Q, both ends to a tube 1 mm. narrow tube thus produced isthen fused off at its middle point;the portion lying next to that partof the glass tube which has not been drawn out is heated asbefore and is again drawn out, and so on. There is th
. Practical methods of organic chemistry . esize of a pin-head, to prevent itfrom becoming dark in colour. Thesubstance is placed in a small nar-row tube (melting-point tube), madein the following way: A glass tube4-5 mm. wide is heated at one pointwhile constantly turned, in a small,blast-lamp flame, until it becomessoft, and is then drawn out from—Q, both ends to a tube 1 mm. narrow tube thus produced isthen fused off at its middle point;the portion lying next to that partof the glass tube which has not been drawn out is heated asbefore and is again drawn out, and so on. There is thus pro-duced a tube having the form represented in Fig. 51 a. Inorder to prepare the melting-point tube from this a file-mark ismade at the points indicated, the tube broken off and fused atthe narrow end by holding it nearly vertical in a Bunsen 51 b represents the melting-point tube in its natural supply of several dozen of these is made and preserved in aclosed bottle. To transfer to the tube the substance the melt-. FiG. 49. Fig. 50. MELTING-POINT 67 a ing-point of which is to be determined, a small portion of it ispulverised, the end of the tube dipped into it; by gentle tappingthe substance is caused to fall from the upper end to the bot-tom of the tube. In order that it may not form atoo loose layer, it is packed by a thin glass rod orplatinum wire. The height of the layer should be1 mm. and in no case more than 2 mm. To attachthe tube different methods may be used. Theupper end of the tube may be touched with a dropof sulphuric acid; this, when brought in contactwith the thermometer, will cause it to adhere. Itis safer to fasten the tube, just below the mouth, tothe thermometer with a thin platinum wire or arubber ring 1 mm. wide. The substance is placedat the middle point of the thermometer thermometer is now immersed in the sul-phuric acid until the bulb is at about the centre ofthe liquid; the flask is heated with a free flamewhich is giv
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