would take its place, and shew in front, which would mar the beauty of the case by giving it a blotchy and mottled

appearance.

The gateways having been cut, the two halves of the flask are closed, (omitting of course the rag.) by once more squeezing in the vice. It is now removed and placed in an iron clamp and wedged quite tight with suitable wedges, or if the clamps are fitted with screws, the flask is secured in that way, and it is then ready for the vulcanizer.

The next case to be described is one where the plaster model is dried, and then instead of using a wax or metal plate, the model is painted while hot with a solution of rubber in chloroform, and then after softening a sheet of rubber, cut it into a suitable shape and mould to the model; we have thus a rubber plate on which to mount the teeth. In arranging the teeth on this plate, we must attach them to it with wax in the ordinary way, taking care, however, to keep the wax away from the front gum.

This is

When they are fixed and built up at the back, the palate should be coated with a solution of soap, and plaster cast into it, so that it covers the palate and just covers the crowns of molars and points of front teeth; this is to keep the teeth in position while the pink gum is being moulded on. the next operation, aud should be done neatly, using suitable tools and shaped pieces of pink rubber, and coating with the rubber solution any part that may require it, to make the pieces of rubber adhere firmly to each other.

After moulding up the front gum with rubber, the plaster plug may now be removed from the palate, and we may finish the remainder of work by one or other of the following methods.

The first consists in roughing or grooving the front part of plaster model, damping it and then mixing some plaster

and covering with it the front part of model and also the crowns and points of teeth.

When this is hard rubber can be packed round the palatal or inner side of alveolar ridge, when the wax has been removed. The case can now be placed in the flask direct and covered in with plaster.

A second method consists, after the plaster plug has been removed from palate of case, in placing the case under a clamp (see Fig. 8), and inserting in the lower part of flask and building up the plaster around it as previously described. It is now painted with a solution of soap, and the upper part of flask placed in position and filled in with plaster.

When hard and the flask is warmed, it may be separated and the wax removed from the backs of teeth with boiling water, and replaced by more red rubber. It will thus be perceived that the only packing required while the case is in the flask is at the backs of teeth the palate and front gum having been already finished.

Of course the flask has to be heated up or placed in boiling water with a piece of linen interposed between the two halves as previously mentioned, to allow of a parting between them and to ascertain if there is enough material. After this the flask is closed and fastened in a clamp and is ready for the vulcanizer.

The usual time for vulcanizing dental rubbers is about an hour and a quarter at a temperature of 315 deg. Fahr., represented on the pressure guage by 100 lbs. A slightly lower temperature and the vulcanizing prolonged to one hour and a half is, I think, preferable; it makes a stronger and more elastic piece, not so brittle. About half a pint of water should be put into the Vulcanizer when placing the flask in. The lid should then be screwed down and the gas turned on to heat it up. When the time mentioned for cooking has expired and the gas is turned off the heat should

be allowed to go down gradually until no pressure or degree of heat is indicated by either guage or thermometer. The lowering of the temperature may be accelerated by allowing cold water to trickle over the boiler, which if of copper or gun metal will not be harmed, but on no account should a cast iron boiler be treated in the same way, as it would in all probability crack it.

The boiler lid is now unscrewed, the flask taken out and placed in cold water.

STRENGTHENERS-METAL.

The insertion of these in rubber plates is a matter that requires some amount of care and consideration in order that the strengthener itself may not prove a source of weakness to the case.

For an upper piece of work the strengthener should be on one side of the vulcanite, preferably on the inferior palatal surface. The rubber therefore would be between the strengthener and the surface of model, and thus any pressure in the molar region would have to overcome the resistance of the strengthener before acting on the rubber plate, whereas if the strengthener was next the model on the superior palatal aspect, the strains of mastication would be upon the vulcanite first, which would yield and be liable to break.

Neither should it be placed (I am speaking of upper cases) in the centre or substance of the rubber palate for such only weakens the case. Exception to this rule must, however, be made in the case of perforated strengtheners which must have rubber on either side of them.

An upper strengthener ought to conform somewhat to the shape of the palate, and should be swaged or struck up on a zinc or type-metal cast representing the palate of the case when properly waxed up. No. 7 plate is strong enough when it is made tolerably broad as the majority of upper strength

eners should be, but if a narrow one is used, then it should be stronger, that is, thicker in proportion, say No. 8 or 9, plate guage.

Strengtheners should be stronger in the centre than at the extremities, and have loops or pieces of wire soldered on to retain them firmly in the vulcanite.

The edges should also be bevelled with a file that the rubber may overlap, so that when the case is finished up the vulcanite will come flash with the surface of strengthener.

When the case is in the flask and ready for packing-in the rubber the strengthener should be placed in position in the upper part of flask, or in other words, on the inferior palatal aspect of case, and then the rubber packed over it, the case being then further packed in the usual way.

In some cases of prominent bite, or where the pressure of mastication comes forcibly on the rubber at the back of front teeth, with a tendency to force those teeth out, the strengthener may be swaged up so as to form a biting surface by stamping it up to a die representing the points of the opposing lower teeth.

This is trimmed up and loops soldered to it. In this instance the strengthener should be sunk, by being made warm, into the wax palate of case and fitted to the bite before the piece is put into the flask.

Another form of strengthener, and one peculiarly adapted for a case with a very thin palate is made in the following manner. (Figs. 11, 12 and 13.)

Fig. 12.

The first shows model to which a thin matrix plate has been adapted, and along the alveolar ridge wax has been moulded. A zinc die and lead counter having been obtained, a piece of No. 7 plate, either dental alloy or gold is struck up as in

Fig. 12, and holes drilled in it. This strengthener is now placed on the model as in Fig. 13, and the plate is ready to mount the teeth upon.