Abstract: A silicon nitride ceramic possessing excellent strength of the surface, including a silicon nitride and a rare earth oxide compound and being characterized in that the ratio of the transverse rupture strength, at room temperature, of the fired surface used as a tensile surface to the transverse rupture strength, at room temperature, of the worked surface used as a tensile surface subjected to working so as to have a surface roughness of R.sub.max 0.8 .mu.m or less is 0.7 or more, and the strength ratio is satisfied even when any portion besides the fired surface is utilized as the tensile surface to be worked to have a surface roughness of R.sub.max 0.8 .mu.m or less. The present invention also provides a process for producing a silicon nitride ceramic including the steps of: (1) mixing .alpha.-Si.sub.3 N.sub.4 powder and .beta.-Si.sub.3 N.sub.4 powder to obtain a raw material powder which satisfies the formula 0.05.ltoreq..beta./.alpha.+.beta..ltoreq.0.50, in which .alpha. refers to the weight of .alpha.-Si.

Abstract: A multi-room type air-conditioner includes one outdoor apparatus and two or more indoor apparatuses which are respectively connected to the outdoor apparatus through respective electric expansion valves. The number of the indoor apparatuses to be connected is detected by a connection number detecting device, the closing degrees of the electric expansion valves are determined by the ratio of the loads of the respective indoor apparatuses and the frequency of the compressor is determined by a predetermined algorithmic expression of a relation between the loads of the respective indoor apparatuses which includes the rated capacities of the respective indoor apparatuses. Thereby, the air-conditioning operations for heating and cooling desired areas are optimally performed.

Abstract: A process for producing silicon nitride ceramic articles is disclosed, which includes the steps of preparing a mixed powder of silicon nitride and a sintering aid, shaping the mixed powder, sintering the shaped body, machining the sintered body in a desired shape, and then heating the machined sintered body in a temperature range from 550.degree. C. to 900.degree. C. in an oxidizing atmosphere. Alternatively, after the sintering, the sintered body is heated in a temperature range from 950.degree. C. to 1,400.degree. C. to crystallize an Si.sub.3 N.sub.4 grain boundary phase, the crystallized sintered body is machined into a desired shape, and the machined body is heated in a temperature range from 550.degree. C. to 900.degree. C. in an oxidizing atmosphere. Thereby, silicon nitride ceramic articles having high strength at room temperature can be obtained.