 The value of the Avogadro constant was first indicated by Johann Josef Loschmidt , who in 1865 estimated the average diameter of the molecules in air by a method that is equivalent to calculating the number of particles in a given volume of gas . This latter value , the number density <formula> of particles in an ideal gas , is now called the Loschmidt constant in his honor , and is related to the Avogadro constant , NA , by 
