The distinction between lutes and cements is not always very obvious. As a general rule, however, a lute is a cement used for connecting, temporarily, the parts of a piece of apparatus or for coating and protecting apparatus that is to be exposed to intense heat.

Lutes for joining apparatus may be needed both for low and high temperatures; for acid or corrosive liquids or vapors, or for those which are easily resisted; and the operator must exercise good judgment in this respect if he would secure success. The lutes described in the following paragraphs afford an abundant variety for most purposes. Those who have occasion to make extensive use of lutes are recommended to read carefully the chapter on this subject in Faraday's "Chemical Manipulations," a work which may be old but can never be entirely superseded. For the joining of tubes of glass or metal, the rubber tubing, which, at the time Faraday wrote, was almost unknown, now affords a cheap, simple, and effective means, - far better than any lute, - where the temperatures are not too high. When the apparatus has to be exposed to a heat at which rubber will soften or melt, recourse must be had to one of the old-fashioned lutes. The following lutes are employed for making joints which do not require to be exposed to a high temperature: -

Glazier's Putty

This makes a very good lute for many purposes, and is frequently used for covering the stoppers or corks of bottles containing strong acids. But owing to the fact that glazier's putty is made with carbonate of lime (whiting) it is not well adapted to this purpose. If the acids come in contact with the putty the carbonate is decomposed, and the resulting gas forces off the lute and renders it worthless.

Fat Lute

This is similar to putty, but instead of whiting finely powdered clay is used. The lute should be well beaten and mixed, as upon this depends its excellence. The clay is not acted upon by any of the common acids, however sti'ong, and the lute is therefore well adapted to closing joints, etc., when these liquids or their vapors are in use. Before applying this lute to a joint the glass should be wiped perfectly dry, otherwise the lute will not adhere; and if the joint is to be made very hot, the lute should be held to its place by strips of bladder or even of linen. The oil used is the best drying linseed-oil, and the clay is pipeclay.

Strips Of Bladder

A very excellent means of joining tubes is to wind strips of bladder round the ends after they have been placed in position. The bladder should be soaked in water until soft, and if smeared with white of egg it will be the better. For all vapors except corrosive acids this makes an excellent joint.

Plaster Of Paris

This may be used occasionally for making joints tight either at common or moderately high temperatures. For the best methods of selecting, preserving, and preparing it, see the article on Plaster of Paris. When applied as a lute it may be made perfectly airtight by coating it with paraffine oil or wax. When it is mixed up with very thin glue instead of water it takes a longer time to solidify, but ultimately makes a much harder and stronger cement. When prepared with water alone it may be raised to a dull red heat without injury, but if mixed with organic matter (oil, wax, glue, etc.) it will not support so high a temperature unchanged.