We reconsider the problem of the difference between the abundances of carbon and oxygen in galactic planetary nebulae (PN) derived from the intensities of the recombination and collisionally excited lines. This discrepancy can be explained by an inhomogeneity of the PNe and an overestimation of the weak line intensities. The formulae for calculation of the nebular line P intensities in presence of both temperature and density fluctuations are given. The intensities of the forbidden [OIII] lines, the CII, CIII and CIV recombination lines and the CIII] λ 1909 UV intercombination doublet for different values of the mean electron temperature T₀ in PNe and the rms temperature variation t², are calculated. Results of these calculations are used to find F the values of T₀ and t² which allow to provide the best fit of the observed and calculated line intensities (taking into account the observational errors). In most cases, the obtained values of T₀ appear to be significantly smaller than ordinarily used for I the abundance determinations T_e([OIII]), while t² < 0.16. The carbon and oxygen abundances for more than 70 PNe are calculated. For these PNe average chemical abundances are evaluated separately for nebulae of type I, II and III. For the first, we found C/H=6.67·10^-4 cm^-3 and O/H=5.74·10^-44 cm^-3. For the second they are C/H=8.94·10^-4 cm^-3 and O/H=6.36·10^-4 cm^-3. For the third we obtained C/H=3.94·10^-4 cm^-3 and O/H=4.79· 10^-4 cm^-3. Results of the fitting of the line intensities for the NIII λ4640 and NIV] λ1486 lines are also given.