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tokens: attention paid to the cryogenic solids of astrochemical relevance, it should be possible to verify additivity given a sufficiently large and varied number of measurements on ices. For our initial study with amorphous ices, we have adopted the method of bond contributions and the values of Denbigh ( 1940 ) to calculate R M values. Bonds used (and contributions to R M in cm 3 mole −1 ) were as follows: C–H (1.69), C–C (1.25), O–H (1.73), C–O (1.51), C=O (3.38), C=C (4.16), and C≡C (6.4). For example, from the five bonds of CH 3 OH, one finds R M = 3 (1.69) + 1.73 + 1.51 = 8.31 cm 3 mole −1 . Figure 7 compares calculated R M values for our compounds to our measured values for amorphous ices. The correlation between the observed and computed values is obvious. Admittedly, the agreement largely reflects the fact that the bond contributions used are from liquids, for which Figure 4 (c) already shows reasonable agreement with our work, but nevertheless the linearity of Figure 7 for ices is gratifying. Figure 7. Molar refractions measured for amorphous ices compared to values computed with a bond-additivity approach. All values are taken from Table 3 . 4.3. Isomers and Molar Refractions Isomeric pairs of molecules in which the number of each type of bond is the same provide another check on our results. For example, since the isomers propionaldehyde (propanal) and acetone each have six C–H bonds, two C–C bonds, and one C=O bond then on the basis of additivity the two compounds should have about the same R M value. Table 4 shows that for this pair of compounds and five others, the R M values are within about 1 cm 3 mole −1 for our 15 K measurements on amorphous ices. A propagation-of-error analysis using Equation ( 6 ) leads to uncertainties in R M of about 0.1 cm 3 mole −1 for the smaller molecules we studied up to about 0.4 cm 3 mole −1 for the larger ones. This implies that the values in Table 4 for isomers are within or almost within experimental error for our ices. Table 4 Molar Refractions of Six Isomeric Pairs Note. a The units of R M are cm 3 mole −1 . Values of R M are from Table 3 . 4.4. Comparisons to Earlier Work The lack of published laboratory data corresponding to that in our Tables 2 and 3 makes it difficult to compare our results directly to those in the literature. To two decimal places, our n for amorphous methyl formate at 15 K is 1.32, near the 1.30 at 20 K reported by Modica Palumbo ( 2010 ). Our n for amorphous CH 3 OH at 15 K is 1.296, roughly the average of the 1.257 and 1.33 at 20 and 25 K, respectively, already published (Luna et al. 2018 ; Sciré et al. 2019 ). In an earlier paper we combined room-temperature data for methyl propionate and n for the corresponding amorphous ice to estimate ρ for amorphous methyl propionate at 16 K (Hudson Mullikin 2019 ). The estimated density value was 0.702 g cm −3 , in contrast to the measured value of 0.764 g cm −3 reported here. The densities we determined for amorphous (15 K) and