Frederick Guthrie on Euteoda. 473 



Fused mixtures of nitre and the variable salt, which are 

 richer in the salt than the eutectic alloy, give up that salt on 

 losing heat ; those which are poorer give up nitre, until in 

 both cases the eutectic alloy is reached. The eutectic alloy is 

 the exact analogue of the cryohydrate; and just as in the 

 preceding memoir curves were drawn showing, on the one 

 hand, the temperatures at which ice separates from salt-solutions 

 of various strengths, weaker in salt than the cryohydrate, and, 

 on the other, the temperature at which the salt, or occasion- 

 ally a subcryohydrate, separates from such solutions as are 

 stronger in salt than the cryohydrates, so, doubtless, the ana- 

 logous curves could be drawn connecting the temperature at 

 which solidification begins with the fused mixtures of salts 

 (see § 223). And, again, as there are always two strengths of 

 an aqueous solution, one stronger and the other weaker than 

 the cryohydrate, which are both saturated at the same tempe- 

 rature below zero, one in respect to the salt and the other in 

 respect to the ice, so with these fused mixtures there must be 

 two such mixtures, from both of which a solid begins to sepa- 

 rate at any given temperature between the melting-point of 

 the most easily fused constituent and that of the eutectic alloy. 

 § 223 is devoted to the tracing of this inquiry. 



§ 218. The analogy is so perfect between the two groups of 

 bodies (eutectic alloys and cryohydrates), that we may look 

 with great confidence for homologous phenomena. The freez- 

 ing mixture, or cryogen of my former memoirs, has its perfect 

 counterpart amongst the salt-alloys, as follows : — 



A layer of powdered nitrate of lead (46 parts by weight) is 

 placed in the bottom of a wide test-tube. On this is laid a 

 thin disk of mica, having a hole in the middle. A thermo- 

 meter is fixed through the hole so that its bulb is half covered 

 by the lead salt. Upon the mica, and so surrounding the top 

 of the bulb, is piled some powdered nitre (54 parts). This 

 tube is plunged into a bath of melted nitre, which is heated 

 some few degrees above its melting-point, and until the ther- 

 mometer registers about 250°. The thermometer is then used 

 as a stirring-rod to mix the two salts. The mercury sinks to 

 207°, and remains pretty constant for a time at that tempera- 

 ture. The contents of the tube liquefy, and solid nitre is 

 formed upon the outside of the tube. Now substitute ice for 

 nitre, and perform the analogous experiment. The nitrate of 

 lead and ice in the inner tube will form a freezing-mixture 

 when stirred together, and this will cause ice to form on the 

 outside, if the melted ice (water) in contact with it be not too 

 warm. 



§ 218a. Just as the eutectic metallic alloys had been nearly 



