The fraction of bosons, f(t), is obtained from a standard equilibrium theory in terms of the chemical constant (t) by f(t)=1-[1+ (t)] - 1 / 2 . Here t=T/T * and T * is the scaling temperature connected with the binding energy of bosons 2Δ by Δ=k B T * , independent of magnetic field together with f(t). The transition temperature t c (H) is obtained from n 0 f(t c (H))=n c (H)=n c (0)[1+γH μ ], where n c (H), the critical density for superconductivity is assumed to be a power law. In the cases where t 0 =t c (0)=T c (0)/T * is small the chemical constant can be approximated by (t)=1/(βt) and t c (H) can be calculated. This is equivalent of knowing H c 2 (y), given by H c 2 (y)=H c 2 (0)[(f(t 0 y)-f(t 0 ))/(1-f(t 0 ))] 1 / μ ,y=t c (H)/t c (0). For several heavy fermions (β=13.7 and T * =150K) data on the quantity μ - 1 ln[H c 2 (y)/H c 2 (0)] fall on the same curve. For Tl2201 we obtain μ=23.